Why can't I let it happen?

Ok so me and my bf going on for just over a year tried doing a threesome multiple times. He told me he wants it to be equal all around except for kissing. I told him I'm not sure but we can try. So we tried and I started freaking out and ran and cried after I said stop. He honestly Dont care if it happens I just am bi and crave a girl sexually sometimes. So he does it to plz me but he feels left out since I'm not fully ok with them having sex. I Dont know why I freaked out. I think it has to do with something from our past. It reminds me of that somehow. I trust him with her and no one else cuz I Dont want a stranger. I want it to be someone we can trust. I trust him no matter what but I Dont understand why this happened like this. Has anyone else experienced this or something similar??? I want it to happen again I just fear I won't be able to go through with it. Or something bad may come out of it if it happens again.