I recently discovered that I've always wanted to see my gf with another guy
That can't be real. Have you ever discussed it with her?
I've tried to but she doesn't think it is a good idea
It's okay to want to live out a fantasy but it's important that your partner is okay with it. If she's not then you have to respect her wishes.
I fantasize that she's getting married to a new bf soon.
While your fetish is legitimate, I believe it wouldn't be if your girlfriend keeps refusing such things. Have you thought how uncomfortable it could make her? If you really want this kind of relationship, perhaps it's time to say goodbye in order for you to be happy.