Feelings of guilt - how wrong is what I did?

Well it was wrong, but only because you could potentially hurt a lot of people, one of them being yourself. Idk if you actually like this guy or not, but he sounds like maybe he has some trouble committing to people and being loyal, and i doubt you would be an exception. To redeem yourself, forgive yourself. All in all, not the worst thing in the world. But you don't want to be the one causing pain to people. And this guy isn't helping you in any way so there's no reason to keep it up with him.