i think the early arrival of any baby makes you feel 'robbed'. I don't know if this is exactly how you feel, but I know when it was the boys birthday it just made me tearful, thinking how early they were and going through NICU etc, the experience was just so unreal, it doesn't feel fair, missing out on all those experiences you normally have with a new born.
books I've read about premmies say it can take years to deal with those feelings. I guess talking about it does help though.
Chin up! You have 2 beautiful boys and number 3 on the way. I'm sure his birthday will be wonderful