Ok - OP, I get you. I do. It isn't about the wedding or baptism or whatever, it is about feeling like the people you most value in life value & appreciate you in return.
Let's face it, usually people put the friends and family they value the most in their wedding party. It hurts when you feel like someone doesn't value you as much as you value them.
Now, I've been in weddings and it does kinda suck but that is besides the point. That has nothing to do with the OPs feelings.
She doesn't feel valued by her friends/family and the wedding is one way they are 'showing' her this. Now I'm sure these people DO value the OP, very much, but it hurts when you are looking for that validation and don't receive it.
My cousin had a baby last year. This cousin is the guardian for my children. Her sister is someone who doesn't have a maternal bone in her body, she doesn't like kids and certainly doesn't want any. I love her but she is immature and unreliable. Guess who is the baby's godmother? Not me. It hurt.
As for getting over it. You just do. It is about being a grown up and realizing that this choice doesn't mean that they don't like you or value your friendship. It is about knowing your worth with out being picked. It is hard but it also comes with age and maturity.
Now, the wedding tomorrow. You go. You support your bf and the bride/groom. Face it these are not people you've known long and there is no reason for you to be in their wedding. Be a grown up. Have fun. Move on.