It sounds as if you may now be feeling the grief associated with your split. You are moving through the stages. At first you feel sort of numb and then when the dust settles, you begin to work through your feelings. (denial, anger, bargaining, depression, acceptance).It's ok to feel sad, depressed, in mourning, down on life etc. You are feeling the loss of the relationship you had with him. I know for me, I feel the same way although I havent officially left yet. I have asked for divorce but I'm in the waiting game until things are final. I know I will be exactly where you are when I do finally leave. Even now, I go through waves of feelling good and optomistic for the future and then very sad and depressed.
Ive been reading a lot of al-anon literature. "Opening our hearts,transforming our losses" is a good book that deals with grief. What I've learned is to allow myself to feel those feelings because it is what I need to heal. I've spent many years repressing the sadness and ignoring the grief of the changed relationship. It didn't happen overnight and Im sure your situation didnt either. My AH slowly became a different person and it IS sad that he is no longer the man I married. It is OK to allow myself to feel that sadness and mourn the relationship and the person he was. It is OK for you to do so too. Be gentle with yourself and give yourself time to heal. Its way too soon to be able to tell if your relationship is going to work. Ive read to give it at least a year of sobriety before attempting to rekindle things. More will be revealed.Just focus on your own recovery and whatever will be will be.