I had a similar experience after my daughter was born and I even posted a similar topic: viewtopic.php?f=14&t=38985
I received lots of replies from people who felt the same way. I was so happy to have her, but at the same time it seemed to hit me all over again that I was missing all of those baby moments with my son. I had a really hard time for quite a few months. Just when I started to feel better I also went on Zoloft to treat my PMS and I started feeling much better. I was reluctant to try Zoloft because I felt that I needed to work through my emotions. But, I desperately needed it to treat my PMS and it really helped with those symptoms. As a positive side effect, I feel mostly back to my normal self the rest of the time too. My loss is always with me, and I still have some hard days but I am able to function normally and enjoy my daughter and I don't worry about negative effects on her because I am not so emotional all the time. I think that this long after your loss, if you're feeling stuck you should look into something - whether it's medication, counseling, support groups or whatever. I understand what you're going through, and I can assure you - it is possible to be happy again.