Tiana, dear one,
My heart goes out to you. I am so sorry for the pain you are experiencing. Truly.
Even though you don't know me well, we are sisters in Christ and I hope you will allow me to speak the truth to you in love.
Do not marry until this issue is settled. If you take a vow with Bob to "love, honor and cherish, forsaking all others" that's what you are committing to do. And you are not currently able to make that committment with a clear conscience and with your whole heart. So don't. From what you've written, I'm hearing that Bob thinks he is signing on for a traditional, monogamous marriage, while you've learned that you love John as well as Bob.
There is no way out of this dilemma; you must work through it. At some point, you will have to deal with it, whether it's now or 30 years from now, you WILL have to deal with it. And sooner is always better than later. Take my word for it; I know whereof I speak.
But you don't have to deal with it alone. You have the great Counselor on your side, and you can trust Him. So talk to Him. Tell him all about it. Pour your whole heart out to Him. For as long as it takes. And then listen to Him, and do what He leads you to do. If you do that, this will turn out alright.
Good luck to you, SisterWoman, whatever you decide to do. Be strong, be just and fear not.