Relationship Seperation Anxiety?

I was diagnosed with Borderline Personality Disorder, and it has become really pronounced since I started dating this guy. We have been dating almost a year, and my disorder has gotten progressively worse. BPD's most inhibiting characteristic is seperation anxiety. So, I have a very hard time when my boyfriend leaves. I feel like he forgets me when he leaves, and doesnt think about me, and perhaps doesnt even love me. I feel like he will just find someone else in his spare time (and I know that he is somewhat of a liar, which makes this even harder to put out of my mind). Thus, I want to spend every minute of every day with him. And when he wants a guys night, I take it very personally and feel like he's abandoning me. Now I KNOW deep down that that isnt true, but that is how it feels, and I cannot seem to make myself realize that it is all in my head. But this gets old to him, and he wants to break up. He cannot take the fights we get into because of this anxiety, and he does not seem to understand that it is a chemical imbalance - I dont WANT to be like this! How can I make this situation better? How can I be okay with letting him spend time without me? How can I improve my trust with someone who has let me down so many times in the past? And most importantly, how can I keep him, and prove to him that I will get better?