Thank you for your questions. It sounds like you have put a lot of thought into what doesn't seem to be working very well in your life and how you would like things to be different.
Your first concern is about reducing the frequency of unwanted thoughts. There are lots of different kinds of approaches for addressing this problem depending on what type of thoughts you're having and how they are interfering in your daily life. There are three general strategies I would recommend to start with.
1. In my experience, we often find ourselves pushing against unwanted thoughts and feelings and trying to force them to go away. This usually causes those unwanted thoughts to grow and become harder to deal with instead of making them smaller. On top of that, we then begin to have unwanted thoughts about the unwanted thoughts! Instead of continuing to try to fight these thoughts, I think it is more helpful to learn to "unhook" from them, allowing them to come and go. Once we have unhooked ourselves, we are free to focus on whatever it is we want to be doing instead of wrestling with those thoughts. Here is a short video describing what I mean: https://www.youtube.com/watch?v=rCp1l16GCXI
So, what would this look like in real life? Imagine yourself studying for an exam. As you are studying, you think, "I'm going to fail this exam." Instead of getting stuck on that thought and starting to panic or become distracted from your work, you might create a little separation between yourself and that thought by saying, "I am having the thought that I am going to fail this exam." This labels the thought as "only" a thought, rather than as a true fact and can help remind you that you exist separately from your thoughts. Then you might say, "Even though I have had this thought, I am going to keep studying," and return your attention to whatever you were trying to focus on.
Of course, this is much easier said than done. It usually takes a lot of practice to be able to do this consistently, especially with really noisy or powerful thoughts. Practicing mindfulness can help you learn to control and shift your attention more easily. There are many online resources and apps for a smartphone which might be helpful to explore. Here is a guide to get you started: https://www.mayoclinic.org/healthy-lifestyle/consumer-health/in-depth/mindfulness-exercises/art-20046356
2. A second strategy for reducing unwanted thoughts is to notice specifically what your unwanted thoughts tend to be about and what effect they tend to have on you. For example, a person might notice themselves thinking, "I'm never going to get this job. There's no point in trying," every time they sit down to do a job application. That person might take a step back and notice that when they have that kind of thought, their next step is usually to give up on the application and go do something else. They might wonder, how does that help me? Giving up on the application doesn't get that person any closer to a better job, but it does prevent them from having to sit with feeling so down about themselves or from actually being rejected for a post. This person can reduce unwanted thoughts about failure by confronting the underlying reasons for their thinking - in this case, maybe thinking they can't handle feeling bad or being rejected. The reason for a pattern of thinking might not always be immediately obvious, but we don't do anything for no reason. Allowing yourself to be curious about why you think the way you do and how that pattern serves you in some way may help you to change it.
3. A third strategy for reducing the frequency of unwanted thoughts is to practice interrupting them and replacing them with more helpful thinking. Most of the time, unwanted thoughts are either completely not true or they are exaggerated versions of what is factual. For example, the thought "I never do anything right" is probably not supported by factual information. A truer version of that thought might be "I sometimes fail," or "I sometimes make mistakes." There is no need to overcorrect and say, "I always do things right," because that would also be an exaggeration. What we could do is notice that "I sometimes make mistakes" also implies "I sometimes get things right," which is both encouraging and probably supported by evidence.
For another example, let's imagine ourselves thinking, "I hate myself." That might feel true in the moment. It might even really be true. It wouldn't help to contradict that thought by saying, "I love myself," because that wouldn't feel sincere. Instead, we might try focusing on a part of ourselves that isn't completely detestable or an action we can take. For example, we might say, "I am kind to my friends," or, "I can keep going, even when I feel this way." It may take a few tries before you find the approach which works best for you.
Your second concern is about acquiring unwanted behaviors such as procrastination. The good news is that picking up habits isn't as simple as "monkey see, monkey do." If that were true, you'd be able to ditch a bad habit by watching someone do the opposite and convincing yourself to do that instead! There are two things I recommend as a starting point for what you have described:
1. Just like with unwanted thoughts, it is important to understand how an unwanted behavior is benefiting you, however indirectly. Nothing happens for no reason, and every problem begins as a solution to some other problem. Common reasons for procrastination include fear about success, fear about failure, lack of interest or enjoyment of the activity in question, and avoidance of boredom or discomfort while doing an activity. Addressing the reason(s) for the unwanted behavior eliminates the need for the unwanted behavior, and it can be replaced with something more effective.
2. Reward is more motivating than punishment. Our response to our own unwanted behaviors is often to punish ourselves with self-criticism and shame, but this rarely leads to long-term change. If we are training a dog, it isn't very helpful to shout at it for jumping up on people. It is more effective to teach the dog what we want it to do instead and reward it for the desired behavior. Humans are just like any other animal in that we learn this way, too. It might be helpful to learn more about reinforcement (things which make a behavior more likely to happen) so that you can apply them with yourself. I recommend Karen Pryor's (short) book, Don't Shoot the Dog! as a starting point.
Your third concern is about changing your current mindset to be more like a previous one. Without knowing exactly what you're talking about or what has happened to change your mindset, it is hard to give specific advice. What I can say is similar to what I outlined above for unwanted thoughts and behaviors: understanding the reasons for your current mindset will be key to changing it. Here are a couple of examples to illustrate.
Let's imagine an outgoing person with many friends. Over the course of a couple of years, this person experiences some hurts and disappointments in friendships and dates someone who betrays them. They begin to think that others will always let them down and they become more guarded with the people in their life. They withdraw from their friends and stop returning calls, so their friends stop calling. This person now thinks of themselves as totally alone and adopts a mindset that they're better off without any friends. From the outside, we can see that this mindset, for all of its drawbacks, started as a way to protect this person from the pain of betrayal and disappointment.
For a second example, let's imagine a person who believes that anything they want will come to them and expects others to do things for them. And why shouldn't they? Perhaps this person has grown up in an environment where they haven't had to put in very much effort to get what they want. As a result, perhaps this person thinks that things will "work themselves out" and therefore doesn't put much work into problem-solving. In conflicts, they expect others to apologize and make things right. This kind of entitled mindset has many disadvantages, but it protects the person who holds it from having to take any accountability for their own life or do the difficult work of self-reflection and improvement.
It is important to remember that a problematic mindset or pattern of thinking formed over the course of a few years is not going to disappear in an afternoon. It takes time and a lot of effort to change situations like these, and it can be frustrating and discouraging, especially if we're trying to do it all alone. You say that these struggles have been happening for the last two to three years, which makes me wonder if there might be some other factors like stress, anxiety, or depression going on. If you can, I would recommend that you speak with a counselor or therapist to help you better understand what's happening and how to move forward. You mention your academic life; it might be worth finding out if your school offers any resources for counseling or similar student support. If not, perhaps an internet-based therapy service like BetterHelp or another platform could help.
Thank you again for your questions. Whatever you choose to do, I wish you good luck.