Have you read
this thread in Tilted Sexuality? The thread's OP is a guy, who is the boyfriend in a similar situation to yours.
I posted in that thread (post #47, page 2) and what I said there is my opinion on this subject, but to save you searching for it, here is an excerpt of what I told this guy whose girlfriend was jealous of his best friend ( a girl who was kind of his "what if" girl too):
" I don't like friends who were exes. If someone wants to dump me for it go ahead. My current SO has a friend who is an ex and he was with her for a year. I trust him when he tells me that he would never do that to me. I have never said to him I don't want you to be friends with her. But it's in my head. I don't like it.
I would be fine if she was a friend and only that. But he had a relationship with her. Luckily for me they are in different countries so I don't have to face it that often.
In your case, you see her often, talk to her all the time, and there is some sexual tension. If you tell your girlfriend that, how do you think she would feel? Maybe you should turn it around and put yourself in her shoes. If she had a male friend who she had dated for a time, and when you saw her together with him everything was all so palsy and touchy and giggly, how would it affect you? (Not assuming that this is what you do, but if you're good friends then you must certainly laugh together, rib each other playfully and act like good friends occasionally, just translating what your GF will see)
I'm not saying she shouldn't accept it. But the fact that even other people who see you together say you look like a cute couple suggests that the way you act together is probably not a "strictly friends" type of situation.
I know this is one of those situations where you feel torn between friends and girlfriend, but if you want to be with her seriously (your GF), then you should think whether you could tone things down with your friend to not cause her distress. Yes it's your friend, that pal for life, I feel the same about my friends. But your girlfriend, if you feel that way about her, could also be for life, your soulmate. Or not. "
I really understand your situation. I don't think you should have to just grin and bear it. You should be able to talk to your BF about anything that's upsetting you, and if he's serious about you, he will try on his part to take your feelings into consideration and maybe change his attitude with his "friend".