Oh boy, this is a tough situation. A piece of advice, nice guys usually steal other people's girl friend, unless her boy friend is a jerk, I mean technically, they are just dating, so, there is nothing wrong about stealing other people's girl friend, but most good decent men is considerate enough for other people's feeling, to not do something like this.
I know, I've been in similar situation before, there was this girl I really like and she really like me to, but, she already got a BF, which they have been "just dating" for 10 years. It was one of those relationship where she is just going through the motion of things and she got no feelings for him.
Normally, when seen people in a relationship like that, I would have told him/her that they need to be honest with them self and to each other, about their feelings and decide where they stand and what are they going to do about it next. Then decide what to do next. But this time, consider my unique position in this story, I've decided to keep my silence. I also decided to start keeping my distance from this girl, been considerate for her BF's feelings. I figure if it is meant to be, then after they broke up, mother Satan (I'm a Satanist) will still make sure we meet up again in this life or the next, after they broke up, if it is not meant to be, even forcing it now, would still ended up in break up, later.
My suggestion for u is, 1. Don't leave ur boy friend for him, if this other guy truly is a good person, it is unlikely he will want to steal other people's girl friend. So, if u dump ur BF for him, most likely, it would just drive him away. 2. Decide where u stand with ur current BF and decide what to do about it. If u choose to stay together, stick with this guy, if u choose to break up, then break up. 3. If u choose to break up, (not dumping ur current BF for the new guy) then if it is meant to be, u and this other guy u like will still have a chance to be together, if it isn't meant to be, forcing it is no use. 4. If u choose ur current BF, then grow distance with the other guy, if it is meant to be, later in life, u and ur current BF will still break up and u will still meet up with this other guy and got together. Or otherwise, u and this other guy, will still be together, in ur next life.