I feel the same way ... . I am a shell of who I used to be with my stbexw. Going out with friends I still have a difficult time - especially the friends that were our "couple" friends. I go through the motions but always find myself leaving early as the memories come back and make me feel empty. I guess a part of it for me is that I truly beleive the two become one in marriage - so now a major part of me is missing. :'(
I feel exactly the same, i've could have written it word for word. It makes you feel empty, and it's just weird. Because seeking replacement (new girl) isn't the answer. Before the girl I assume you felt 'whole' as well (purely a guess)