Are you just not attracted to him? Does he open up about himself, at all?[AH]
What is it, about being the center of his attention, that is leaving you feeling suffocated? Is he clingy, needy, possessive? What are the red flag indicators, that this nice guy, is truly a bad boy at heart and ever so not right for you?

Are you, not ready for that level of commitment, not that you've expressed it, but nice guys, you bring home to mom, type of thinking here, as far as this one goes?

What exactly, is the elephant, in the room? Or are you just not fully recovered from your ex?

Ex#1, sounds like a roller coaster ride. The NYC trip, sounds like a love em and leave em type. If any, what were your other bf's like?

I guess, if there's no spark there, between AH and you, then it's not fair to him, to trail him along, when he can exert his energies elsewhere, kwim?

Perhaps, there's things, about yourself, to conduct a little more soul searching on. For instance, it's not fair to yourself, to completely blame yourself, for the fall out of the ex of 4 years. It takes two. There's something about him, that played a role in all of this, and he led you along, all those years, to just not commit to you. Not about you not trying harder, just sounds like he didn't know a good thing when it hit him, and decided that continuing to sow his wild oats was more of a priority than investing himself, whole heartedly into a relationship with you.