The comments here about grief are so true. It's like a death - death of the relationship but also the hopes and dreams you had. I still find myself shocked that my XABF and I aren't together. He was a major part of my life too and that takes time to adjust to. It does get better though and hurts a little bit less each day. Be prepared to yo yo where you may feel you aren't making progress. You will have stagnant times too but it will move forward. What helped me was not just focusing on me but also reminding myself by us splitting up, I was giving my XABF a chance to stand on his own two feet and sort himself out. If he didn't (and sadly it looks that way) then at least I'm not dragged down by it.
Staying with him wouldn't have helped you or him.