Demetri Volturi.

I imagine that Demetri was a guy who liked to have fun in his human life. He was casual and irresponsible and a flirt; he genuinely had no practice with happy relationships. Oh sure, he had his friends, but deep down, he knew the whole dynamic was based off of rebellion and escapism, not compatibility. As a result, any meaningful connection freaked him out a little bit. 

Then, as you say, his immortal emotional life was characterized by abandoning Amun and joining Aro. To this day, he doesn’t get what happened there. It’s a moment of murky emotional confusion, so he tells himself that his feelings about Amun couldn’t have been real. And, if something that felt so genuine actually wasn’t, then clearly, he must not be capable of commitment at all. (Of course, he works very hard to prove his worth to the Volturi, so he’s not even committed to the idea of being non-committed…)