I greatly struggle with my fear of abandonment. Like you, just because my T proved one day that she won't abandon me, doesn't mean the next day she won't. And yes, I have begged her to stay. I have asked her to promise me she won't leave. She used to respond that she's not going anywhere because she likes our city and her job. That wasn't a good enough answer for me. Just because she's not going to leave our city or career, doesn't mean she won't leave me. Now when I ask, she promises.

I do panic when she leaves for vacation or has to miss a session. I fear something happening to her, or that she enjoys her time away from me that she terminates me when she gets back. I also fear she will forget about me and will double book.

My T has me keep a written list of all the ways she has showed me she cares. Every time I worry, I look at the list and remind myself that she really does care about me. And anytime I need it, I can ask her for reassurance.

There's another thread that talks about transitional objects. Maybe you can ask your T for one. Or ask to borrow something of hers when she goes on leave.