I've so been in your shoes. It is incredibly hard to let go.
I realized that in my loneliness, I was hanging on to a fantasy, not reality. My fantasy was that my AH cherished me and adored me and would climb mountains to keep our marriage together. The reality, of course, was that my AH cherished and adored his other woman --- booze.
I feel like I went through the stages of grieving. It has been incredibly painful with many months of feeling bleak. Eventually, there were rays of sunshine. Now there are whole days of sunshine. Hang in there. Take care of you in the best ways you can figure out. Let yourself grieve, but don't get lost in it. It will get easier with time.