I'm going to tell the bride I'm not going to the rehearsal dinner as a bridesmaid
Why aren't you going to the rehearsal dinner?
I decided that I'd rather do something else.
It's really inconsiderate of you to not go to a rehearsal dinner when you've agreed to be a bridesmaid. If you say you're going to do something, then you should honor your commitment. I think you should reconsider and go to the rehearsal dinner for that reason
My problem is that I don't want to do it. Some things aren't as important in your life as others.
I cannot imagine what is more important than a friends wedding.   Would breaking the trust of your friend really be worth whatever else it is that you want to do?