I think you’ve already figured out that it’s only fair to everyone to make a choice, but you don’t like having to do it. Indecision keeps both possibilities alive for you.
One of my teachers used to say that two relationships are fewer than one. You are not committed to your boyfriend because you are allowing feelings for the other guy. You aren’t committed to the other guy because you have declared yourself to be in a long term relationship.
It would be terribly unfair to your boyfriend for you to marry him under these conditions. You are correct. Unless you settle your feelings, the “what if guy” will keep surfacing in your mind, especially when you inevitably run into the rough spots that every couple runs into over the course of a marriage. Sadly, this almost always guarantees divorce. Instead of working hard on the marriage, a person who goes into a marriage second-guessing often tells herself she made the wrong choice in the first place and leaves.
If you remain ambivalent, perhaps you should see a therapist to come to an understanding of what exactly you mean by “not being ready for marriage.” There may be more to this than you think.
I wish you well.