Definitely been there. The only honest-to-god truth I can tell you is that sometimes, you just have to feel crummy. Sometimes you just need to have a bit of a solo pity-party where you have a good cry, listen to some sad songs or watch a sad movie and just get it out. Eat the junk you don't normally let yourself eat. Normally I say communication is key, but I know in these instances I never used to let my husband know when I was sad for these reasons because there wasn't anything he or anybody could do to fix it, I just needed to be sad and I didn't want to make him feel guilty about the fact he couldn't help or that he was having a good time.
Then, once it's out of your system and you've cried it out of your head (or at least the big chunk of it), you can start to focus on other things - like the positives of how you won't be apart for too many more special occasions. I know people want to tell you to be strong and to think of all the good things yet to come, etc., but sometimes you just need a good cry to help you ease your mind