I think they call this love-bombing these days OP.
It happened to me once, when I was about your age, and it was so upsetting. I was introduced to a man by some friends, we got on well, and started seeing eachother. He was so keen, really liked me, seemed to see wonderful things in me that no one else had - and of course I fell for him, because he made me feel like I was amazing (and that's a pretty irresistible feeling).
Then after 2 months we had a phone call in which I sensed a shift, and 2 days later he dumped me. I was totally stunned, really hadn't seen it coming. And I was sad, because when the person who appears to "know" how great you are suddenly doesn't want you, it makes you feel like crap.
Anyway I bounced back pretty quickly, after the initial kick to my ego. But we stayed friends, as he was part of my larger friendship group. Over the next few years I watched him do exactly the same thing to a succession of women. It was clearly his way of doing things - he wanted his relationships to work out, so he put his heart and soul into them, trying to convince himself each time that this was "the one", then backing off when he realised it wasn't.
I think this may be what your guy is doing. It's really really painful at the time, makes you feel dreadful about yourself, confused and powerless - but the pain will be short-lived I'm sure.