1. Because he is an addict.
2. He's doing these things because he is an addict.
3. He did that because he is an addict.
4. I wish I could just pull that thread that unravels all the pain you are experiencing, but feeling it instead of avoiding it by indulging in your drug of choice (your XABF) is part of getting over it.
The only thing that made a difference for me was accepting my X for who he was and to stop expecting him to behave like a rational sane human being. I was torturing myself with expectations that he was unilaterally incapable of fulfilling while he was still in active addiction and showed no evidence of wanting to recover. The person I "missed' and longed for did not exist. I had made him up in my head.
So maybe instead of "why is this happening" you might ask "why do I keep expecting anything else".