Well, maybe things are have past this station already, but my initial thought is that if he really thinks you are the best thing for him, but the suddenness and intensity of the relationship overwhelmed him(which I think can happen), a solution would be to stop living together but continue the relationship.

I am sure that in your talks the difference between breaking up and stopping living together was clearly stated and discussed, but I feel that in your email it is not.
If it was discussed, why did he want to break up? He should have been able to communicate this feeling to you without making you feel uncertain/hurt/in doubt.

Of course when long-term expectations from a relationship have to match up.

So probably he told you he wasn't sure that he wants to spend the rest of his life with you, right? And now your relationship already progressed a lot and maybe feelings he hoped he'd have or develop, weren't there?

If so, a breakup is probably best and in that case he has probably already moved on from you.