I'm annoyed at my best friend for getting closer to my boyfriends ex
Why does that annoy you?
I'm afraid my friend might steal him from me
I understand, but it's a bit petty to try and control a friend who wants to date your boyfriend's ex. You should let her just be happy.
I'm not petty, but it bugs me. I just don't want her to take him from me.
I think this is a situation that you need to move on from. Give it some time and I think your concerns will fade away.