How do you move past a breakup?
It's not you; it's her, mostly anyways. Together and breaking up, the extremes seem to depend on her moods. So, what do you do with that? How do you stay with someone who is like that? Do you think you could be with this person as long as you toe the line and keep her happy? No, that is not what someone who embraces an erratic and shifting mood wants; they want chaos and confusion and thrive on it. So, what you can do is to sit with the situation, knowing you can't fix it, or fix it here and either accept the life you would have with her erratic and emotional self or get out and look for something that you know has the ability to reciprocate the feelings and emotions back to you. This person doesn't seem capable of doing that; she seems to be consumed with herself.
The best part about what I said was that it puts the choice back into your possession. She does what she does for whatever reason. Your job is to know what is good for you so you can be good and helpful and good for others. If you must constantly be at the mercy of someone's shifting mood, there isn't much left for anyone else. Are you willing to give up your life and what you mean to others to be with her? The choice is yours despite what you might believe today.
You aren't lost. I think you are questioning yourself. She is selfish and can't think of you in all this. If she could, she wouldn't be testing you like this. I believe that you aren't willing to accept the reality that you know what you know about her, and you are going against this knowledge, invalidating yourself, leading to this lost feeling. Lost might mean trying to deny yourself to try and accept an alternative narrative that isn't true.
Trust your gut on this. Trust that what you notice is legitimate and that you can validate and support yourself in this process. The pain here is when you deny yourself and your feelings in the process when you accept these behaviors of someone else, and it hurts you. It's like going to yourself and saying the worst things you would never say to anyone. You are denying yourself when you allow your boundaries to be crossed by her when she comes and goes as her emotions direct her.
Knowing doesn't necessarily lead to change. Learning can help us make a change if we are willing to endure the pain. Pain is inevitable no matter what you choose in life. There is pain in staying in the same situation. There is pain with change. Pain has to be accepted. You can experience pain on the way to something valuable to you versus the pain of denying and hurting yourself. Live according to your highest value, and the pain to do it (such as setting boundaries or not letting this other person cause you harm because it's harmful to others) is worth it.
The real question is, what matters to you and prevents you from living according to that direction? If a relationship isn't helping you grow but draining you, get out. Endure the pain of turning people down that prevents you from living your highest value.