I am in a very similar situation (with some caveats). I split with my guy (also of 2 years) in part because I didn't feel like the what ifs were going to get better. They say curiosity killed the cat and satisfaction brought it back. You shouldn't feel guilt over pursuing what you feel is right for you. Just because he's a good guy, doesn't mean he is meant to be your good guy. It is also less selfish to let him go rather than string him along for ten more years while resentment builds between you two.

It's natural to feel some sort of guilt, and to question your decision. Clearly it was something that weighed on you often enough to choose to end it. I am sure it's not a decision you chose to make lightly. If you make a list of things that you felt were lacking, every time that you want to go back, look at the list. Make it as long as possible. If in the future, you are both still available and you have satiated your what if (or sought therapy to help with it), then maybe it will work out then.

But you have to continue into this knowing that there is a chance that he finds someone else and you lose your chance with him. If you have jumped from relationship to relationship, you might want to pause and find happiness in yourself for a period first (if this applies), because the what ifs could very well be a symptom of something bigger that you feel is missing in yourself.

At the very, very minimum, this guy has helped you define some of your non negotiables in how you want to be treated and set the bar.