Afterwards, I also learn that my boyfriend’s last ex girlfriend, who was the most serious out of his exes, is on the wedding party for the bride. This made me livid because they didn’t even try to make me feel more comfortable about this situation.

Make you comfortable? This isn't about you. She's a good friend of the bride and was asked to be in the wedding party on that basis. That has nothing to do with you. Marrying is a busy endeavor, your comfort isn't on their list of priorities. Nor should it be.

It's unfortunate that you're the odd one out when it comes to the personal dynamics here, but the wedding party was chosen and a few extra choices seem to have been made on that basis. Would there be unwanted tension because his ex is in the wedding party? You're spitting fire over the fact, so maybe it is a good call. Well, good or bad, the choices have been made.