In a relationship but may still have feelings for old crush

Ok. I adore my boyfriend. He is the world to me and I intend to marry him. I wouldn't do anything to hurt him, ever, but I might have a problem.
Before my current boyfriend, I had mutual interest with a really good friend of mine. We never got serious, nothing beyond some flirty texts and talking on the phone and stuff, but my mom decided she didn't like how close we were getting so she forced me to cut ties.
About a year later this friend and I reconnected. At that time I was a month or two into my relationship with my boyfriend, and everybody was aware of where things were at. It's been going well but things are starting to get complicated.
The problem is, my boyfriend would absolutely permit us to have an open relationship. He doesn't want to get cheated on, but he says he trusts me to come back to him and he supports me in whatever I want to do. I recently told him I may be open to trying a threesome, but what I haven't told him is that I think I have unresolved feelings towards this old crush of mine.
I'm not exactly sure what to do. Breaking up is out of the question, it would destroy me to lose my boyfriend. But I don't know how to approach the subject that there might be some extra feelings going elsewhere. Advice is very much needed.
I also posted this in the sex category.

UPDATE: I decided to talk about it with my boyfriend and he is trying to look at it as objectively as possible. He could share me physically, but he won't share me emotionally, so he told me to try and find out if there are any true feelings with substance left between me and my old friend. He also told me I need to be prepared in the event that they are stronger than my feelings for him. We've been together a long time, so it's never been a question of whether I'm committed to him, but he tells me my judgement on him is impaired because of the old feelings. So now I have to talk to my friend about it and hope things go the way I want them to: I want to be able to stay friends, and I want to go on being happy with my boyfriend.