I read your post quickly. I hope I got it all!
I hope this isn't all too assuming. I can Totally relate. I am now 43, and have had four or five major relationships. In two of these relationships, I was utterly preoccupied with being left, constantly. This led to a lot of angst for me and my partners. It led to constant anxiety on my part. My first boyfriend died very young. My second boyfriend left me after 3 years during this time I lost considerable weight and went through umbareable pain.
As I have matured, I have come to deal with these feelings. To own them, and to know that they aren't based on the reality of the relationship at hand.
What I found, and this was key, that these feelings derived from my relationship with my PARENTS - the fact that my father abandoned me and my mother was neglectful. I realized that I was transferring these feelings onto my partners. Did you have a good family life? Is there any possibility that these feelings go back further than your break up?
Also, with my third boyfriend worse case sernio DID happen. He rejected me. We were madly in love, but he didn't want to be poly with an already married woman. He wanted a very conventional relationship with someone his age (who he is now getting married to). I worried the entire two years of our relationship about his leaving. I talked obsessively to everyone about it. However, It happened, he did leave and I'm OKAY - good in fact, in love with someone else and still married. So try not to worry and look to your past.