I can't stop thinking about someone who is not my boyfriend. How can I resolve this?

I’ve been in a relationship for over two years. It has had its ups and downs. He is a wonderful person, kind, caring, but intellectually we are very different. He is just not capable of providing me with the mental stimulation I have grown to enjoy in my friends. When we first started dating, we were both quite the partying type, now he is more into health and athletics and I am interested in academia and seeing the world.

I feel like such a moron because I know a better, more decent guy will be hard to come by. I wish I didn’t feel like the relationship has become stagnant. I have found we barely have any meaningful conversations, and I am very bored when I am with him. As silly as this sounds, it is very painful because I “love” him – I just feel as though the flame has officially burned out. I know it sounds rather ironic that I feel pained by this, when essentially I am the source! He is my best friend, and is one of the only people who has always been there for me. However recently I feel as though he has been departing a bit as well, being much less attentive – it has been quite a two way street in this regard.

We broke up not too long ago, because we had been living together and were not compatible. Moving apart helped keep the relationship going, but I feel back in the same place again. I feel as though I would miss him tremendously if we split, but I have been feeling constant guilt because deep down I know how I actually feel.

I have been under huge stresses in my life currently, and I am worried that my recent “feelings” have simply become an outlet to relieve that pressure. I don’t want to do anything I will regret, but I also know this is totally unfair on my significant other. I’ve been spending time with a friend, and we both have begun to like eachother. Nothing at all has happened, but I feel as though mentally I am cheating on my boyfriend and that is just as bad. I wouldn’t want to be in his shoes.

Is it bad to spend time with someone that I do have some feelings for? I know that I need to decide on my relationship now without being influenced by this, and I can’t help but consider the “forbidden fruit” syndrome going on here.

I’m afraid of taking the plunge and straying away from my comfort zone. It is rather funny because in all other aspects of my life I am very adventurous and independent – here I seem to stumble.

All in all, how in the world do I sort out my feelings? Do I need to “man up” so to speak and just get on with my life? Nothing ventured nothing gained? Have any of you been in this situation before? Is it wrong to continue socializing with this other friend, while I am with my boyfriend, as long as nothing across the line happens?

Apologies for the wall of text – I just wanted to provide enough context to paint some kind of picture.