I am sorry you have yet again found another low time in your life. I too have BPD, & was diagnosed at 21, & am in my late twenties as well.
It took a lot of work for me to move forward & although I often feel pain, even on a daily basis, it can get better. I'm now working on a "Life Worth Living" Most of the time when I am in an emotional state, it's hard but I know to try not to believe what my mind is throwing at me, because it's usually not true. I have a close family member that has done a fair bit of research on the illness as well, & know me quite well, that I can call, & discuss what it is I am going through, & ask, "Is this BPD? or is it me?" "& what can I do about it, how can I change this pattern?"
Perhaps you have already been through this process, I always found doing group therapy REALLY helpful. Especially to see what others were going through that are like me. & getting idea's from one another on how to handle a situation.
I know what it is like when the mind is scrambling through the same thing over & over & over again. Such as the situation with your ex. You know it is not a healthy relationship, but you cannot help keep going back to it, because you feel it is your only connection to having someone that was once close, care for you & understand & support you in some way.
Quite a few years ago, I went cold turkey on a lot of destructive behaviors. & I've SLLLOWLLY been coming back into my own society..creating it rather. There are things I cannot control & never will be able to but being aware of why something is happening, so that I can take a different step. Learn a new perspective has immensely helped me out.
It was PAINFULLY hard to cut a lot of those behaviors, the biggest one, having men in my life, to make me feel good, because I didn't feel good, & having them there was like a drug because I was sooo very alone dealing with agoraphobia at the time. Not feeling capable of living. Seeing a guy, getting that HIT (figuratively), & than being left or used or whatever & always questioning, punching at the air so to speak. Feeling Much Much worse about ME afterwards. Even though I'd stop, it took my mind many years to get away from those feeling's. Filling the void. Figuring out what it is I ENJOY.
Even today, 6 almost 7 years later, I have major anxieties about getting involved with others. Dating. Having a relationship. How I want to be treated. How I need to treat another. Baby Steps. It doesn't all happen at once. Finding your own way. I found that I needed to cut ties with those that wanted me a certain way, so I could again, find my way.