The splitting thing really triggers my anxiety I guess and that's why I find it hard to not text him.
This is a very important insight, and I think it's something to pay attention to- the sticking point that keeps the two of you in this drama. Note- I am not talking about the relationship staying or ending- that isn't what this is about. You are quite clear that you want this relationship. It's that there's emotional drama that upsets you. This idea is about your part in it.
We all have a part in the interactions between two people. One interesting thing that I read about relationships is that we "match" our partners emotionally in some way- we are equally enmeshed in ways. Even though it's your partner who has the BPD and you don't- there's something that is holding the two of you in this situation that upsets you.
You can't change how someone thinks or feels, or their behavior. You can change your response to it. Learning about being "triggered"- we speak of these as if they are something someone else does to us, as if we are passive victims of being triggered. But someone can't trigger us if we don't have the ability to be triggered. This is our emotional response- and we do have control over that.
How do we change that? Change your part, change your reactions. It's not easy work and maybe not perfect but it can make a difference in how you are feeling. Rather than focus on what is going on with your partner and why- focus on your anxiety and learn ways to manage it. It helps to look at feeling triggered not as something that someone does to someone else, but that- the trigger belongs to the person who feels triggered and rather than see this happening as a negative thing, it's an opportunity.
If you feel triggered, it's a signal to stop the interaction at the moment, and learn to calm yourself down rather than react. It's a signal to do some self care. Rather than react with hurt or emotion, calmly say "I think I need a moment to think about this, I will call you back later" then stop the conversation- because you know it's not going to go anywhere good while you are upset, then do some self care- listen to music, take a break, until you have calmed down and feel better.
If you need support with the anxiety, counseling can help. It's the part you have control over. What is the sticking point between the two of you? He is "using" you to manage his own emotions. You are calling him to manage your anxiety- that is also using him to manage your emotions. The key to changing this is not to try to control your feelings by understanding him better- it's to work on being better at regulating your own emotions so that you can stay calm even if he's not.
He may have BPD but that doesn't mean he doesn't have choices or his own decisions. If he tells you he wants to not talk as often, he could actually mean that. If you keep calling him after he says that, consider that a boundary violation- he said to not call him. Although you feel anxious not calling him, he can have boundaries too. He keeps bringing up breaking up which is understandably scary to you but truly, all relationships have this possibility. It's possible that he means it, but if he doesn't- why would he keep brining this up? Perhaps because of the emotional exchange. It's familiar, it gets attention.
When he says "don't call me so often" you can say " well I will miss talking to you but I hear what you are saying and so how about you call me when you feel ready to? I'll look forward to your call, love you, bye" - then you hang up and take care of you.
When he says "I think I need to, or might, break up with you" you can reply "well I'd feel sad about that, but please let me know what you decide. I need to get back at work now. Love you, bye" and get off the phone before you react emotionally.
He says "don't wait for him" - well don't. Neither of you can change your situation right now. He's in classes. You have a new job. This doesn't mean you have to break up or meet someone else. It means- you have a certain amount of time apart- you can do things you enjoy or you can sit around thinking about him. He's going to be in school regardless so this choice is yours to make. So yes, invest some time in the job, and on your own self care.