I want to be in a relationship but only with one person

There's this guy who went to my college, haven't spoken to him or seen him in nearly 4 years, but recently I keep thinking about/slightly obsessing over him. He used to have the biggest crush on me when we were 16-18 and I liked him back but had no experience with relationships and was the most shy person you'd ever meet so didn't show anything back, which I regret. He now has a girlfriend of 3 years and before anyone says anything I'M NOT TRYING TO SPLIT THEM APART OR INTERFERE WITH THEIR RELATIONSHIP ETC.

I think I keep thinking about him because he used to treat me so well, like honestly the stuff you read about in books and see in movies. He knew everything about me and no one's ever made me feel as worthy/beautiful/happy to be alive as he used to.

I've been very confused about my sexuality for years, whether I'm straight/asexual/demisexual/aromantic/somewhere on the spectrum, and I've only recently started *realising* what I want. It's still kinda hard to explain maybe but basically I like guys and can find them good looking, but I can only be more emotionally attracted to them once I've seen how they behave/how they talk/their personality etc. But, I wouldn't want to be in a relationship with them. I'm not sure if that's aromanticism(?) or I'm just insecure and hate myself and awkward and don't want to have a big commitment like that in my life, but when I think of this guy I can imagine being in a relationship with him. I'm SO emotionally attracted to him, I tried stalking his Twitter and LinkedIn to give me the ick and it didn't work. I feel like if I was in a relationship with him I'd feel happy and not insecure about myself, as that's the way he made me feel before. I physically can't imagine myself in a romantic relationship with anyone else. Actually, the more I think about it, he's the only guy I could imagine myself in a s*xual relationship with as well, even kissing.

Obviously he could have changed as it's been 4 years but I see his posts with his girlfriend and he at least seems the same, he's so nice to her and talks to her (in comments etc) like he used to talk to me.

AGAIN I am not trying to break them up or anything as I think they're very well suited and the girlfriend is SUPER nice, but my mind is going crazy and this whole 'not wanting to be in a relationship with anyone apart from him' is obviously unhealthy and there's something wrong with me.

Honestly I reckon you are idolising him and he must have changed quite a lot during those years. Also given the fact that he has a girlfriend, forget about him and focus on finding happiness by yourself, no one will ever make you happy if you aren't by yourself

Is he the only guy who has shown interest in you?