It's not clear from your post just how intimate your relationship with J is, so I'll assume that it's only emotional at this stage, not physical.
Kudos on fearlessly telling your wife your true feelings; how you feel about J is simply the way things are, and the sooner the truth is out in the open, the better. That said, it sounds like you have doubts about your wife's willingness to enter into a poly relationship, or to have you entering into one. The possibility exists that this may simply be something that you and her will never agree on. In that case, you will have to decide whether your relationship with your wife, or your freedom to explore relationships with others, is more valuable to you.
Unless your wife has a complete change of heart on poly relationships, I'm sorry to say that I don't see this situation resolving itself with everyone 100% satisfied with the outcome. You followed your heart when you told your wife your feelings about J, and I hope you'll follow it again if a time comes when you are faced with such a tough decision.
Good luck, and I wish you, your wife, and J all the best.