I'm torn in two, and I swear I'm through...or am I?

Some situations are rough! ...you see, I've been dating this guy for almost two years and he's such a sweet guy, the kind of Prince Charming every girl dreams of and every parent cherishes. We broke up for awhile, and I found myself interested in my best friend since 6th grade, and I felt like I never had before...like electricity was flowing through my body whenever he was around, or even whenever his name came up. We became even closer because we had both ended long term relationships, and one night, I don't know what came over me but I kissed him. Ballsy right haha..the crazy thing is that it felt so perfect..and it was all perfect for awhile, I'd never been so happy in my life...but then his best friend was in love with me, and he felt so horribly guilty which put a huge strain on our relationship...and things weren't so great anymore..I was just so afraid of being alone, and I realized that maybe I should give my ex boyfriend another shot to avoid getting hurt. However, this other guy is still in my mind all the time...whenever I see him, my heart finds a way to keep rebreaking, and I'm still head over heels. I feel terrible because first of all, when I got back with my boyfriend, the other guy began "talking" to his ex girlfriend again (even though she cheated on him repeatedly) and now our friendship has a huge strain...I feel so horribly guilty and I'd never cheat on my boyfriend, but I can't help to want this other guy more than anything...I'm torn in two because my boyfriend doesn't deserve this, and it just doesn't make sense to let go of something that you've had for so long.. but on the other hand..it doesn't really make sense to hold on to something that's not actually there anymore.. and if I do break up with my boyfriend for him, I don't know how to go about getting him back.. I have no idea what to do :(