I would explain exactly what you have said here and say how you feel to your friend. You are allowing yourself to be 'vulnerable' and most people with any kind of a heart would scoop you up and look after you as part of their group. If your friend says 'well come with me' you have an open invitation to go. Go on, just go and have a good time.
Take every opportunity to join these celebrations. Even though they are not your own. Every door that opens - grab the door with both hands. You just never who you might meet, or what opportunities might suddenly turn up. Go and let your hair down. Just go and be happy for those that have gone through the meat grinder. Your turn will come (& you might be inviting them all back again when it does)