Oh my goodness. I understand how you feel. My xabf left a month ago and it hurt so badly that I spent 3 weeks in the worst black hole of my life, sitting on the couch. Mine also owes me money, lots...
When I started making too many noises about not being happy and wanting him to take responsibility for himself, he just moved himself right out. And I went through all the same things you're going through, the "how could he possibly do this to me" questions and the "but he always said he loved me" stuff. Ouch.
You'll get a lot of support on here to keep you strong in your decision. You really shouldn't settle for a relationship where you are in this messed up dynamic and negotiating with an addict is a waste of time. You'll never get what you need. The only way through this, I think, is to start thinking about it all differently. Al-anon has been really helpful to me. Like a refuge in a war zone. And now I'm starting to retrain my thinking so that I never get into another situation like this again.
As for your last question, you probably know the answer. He can do this because he's an addict - read the "classic reading" sticky. And he can also do this because you've made yourself available to be treated like this. I'm really having to look at why I have thought that it was OK to be in relationship with someone who is not fully functional or available, for whatever reason. This is hard stuff to face, but the good thing is that it's really possible to avoid repeating this in the future.
Stay strong, and HUGS!