Just wanted to add my perspective as a man, and as an occasional moron.
This is just my view and I could be wrong and I don't want to upset you, but...
This sounds like a guy who falls in love easily and probably often. He loves falling in love, and gets infatuated quickly. But he struggles to maintain this over time. The thrill wears off. When his head is turned, he becomes infatuated quickly, and this overtakes his ability to be a nice guy and do the right thing.
When he said those nice things to you the other day, he was in his own rubbish way trying to make you feel better about what was to come. He was trying to drop in his excuse that he wasn't good enough for you.
This is the hard bit: He is infatuated right now with someone else. This enables him to act like a complete git, because his infatuation destroys his sense of right and wrong.
So my view is that he is the person you knew, you don't need to question yourself about what you thought of him. You just didn't know he could so easily become infatuated with someone else, and treat you so badly as a result. How could you have known this, until it happens?
Wherever he is now, his new flame has the same thing coming to her, eventually.
The reason I believe this, is because to my shame, this is me, and I have to fight to deal with it. I read mumsnet from time to time in order to give my head a shake and realise the damage this behaviour causes.
His new infatuation will wear off. And when it does he's going to have some very uncomfortable memories of what he did to you.
If his new fling doesn't work out, it is HIGHLY likely he'll come crawling back with excuses. An earlier poster described how men can break up quickly and easily and then a month later fall apart, whereas a woman goes through hell and a month or three later gets on with her life. This is (broadly) true, in my experience at least.
I've read every suggested text that some here have suggested sending. Not one of them would have the desired effect. He will see through them all, as do I. Every one of them gives the power back to him. Please, PLEASE do not message him. If it was me, I'd be delighted to get anything back from you. I would read it, process it in the way that suits me, and then feel happier because now I have the closure I needed.
If you feel the need, write him a long and detailed letter about why he's a git and how badly he hurt you.
Then burn it.
I'm sorry this happened to you, but you're handling it amazingly well. Keep going.