How can I avoid overthinking?

This sounds like a difficult situation with some conflicting feelings - you felt you needed to end the relationship with your boyfriend as he was going through a difficult place and you felt on some level that you were an added pressure on him, in terms of keeping up the relationship. Now you are feeling that you would like to be with him and are experiencing feelings of regret - this is leading to you overthinking about the relationship and everything around him. This sounds really overwhelming to you and is now causing you anxiety. 

Overthinking can be a difficult pattern to break, as it's a vicious cycle of negative thinking that builds upon the last thought. Overthinking goes hand in hand with anxiety, which you have identified as something you are experiencing. The human brain will react more intensely to negative events than it will to positive ones, and breaking up with your boyfriend has been a negative event, which might explain why it has been so hard to stop the overthinking from spiraling.  

You may have heard of Cognitive Behavioral Therapy (CBT), which can be helpful for overthinking and anxiety. CBT looks at how your underlying thoughts inform the way you feel. This in turn impacts how we react to the world (our behavior, such as thinking patterns and our reactions to other people). CBT aims to challenge our thought processes and regain control over our spiraling negative thoughts. It helps you to recognize and question negative thoughts and turn them into more balanced thoughts that don't bring you back into that spiral.

Person-centered therapy combined with CBT may be helpful. CBT works in the here and now to manage symptoms, whereas counseling where you talk about your experiences from your past and present and your role within them can help you to process what you have been through. The counseling room is totally non-judgmental and you will be held in positive regard and there is always empathy present. This kind of environment can be unique for some people - they have never been able to express themselves without feeling judged, or having an opinion or solutions constantly presented. These things will inhibit the ability to accurately reflect on our situation. Reflection is vital for self-awareness to grow. With increased self-awareness, we become better placed to make good decisions for ourselves, whether that be putting in boundaries with certain people or modifying our own behaviors for instance.

Other techniques that might be helpful for curbing overthinking could be Mindfulness, where you apply techniques to recognize that thoughts are just thoughts, perhaps based on a past you can't change and a future you can't predict. With practice you can learn to not attach relevance to all of your thoughts - after all, they are not always factual. Having a more peaceful mind helps you to be present and enjoy what's happening in the here and now. 

Journalling can also be a helpful tool - moving thoughts from swimming around in the mind to getting it out on paper can help to reframe thinking and can also be quite a cathartic experience. Perhaps you could ask yourself specific difficult questions, which might help you to reflect on how you really feel. Sitting with these feelings and having the courage to do so, can sometimes shift how we are feeling and help us to move on.