How old were you when you got together? Could it be that he was just using you for sex and now he is done? I know alot of older guys take advantage of girls and move on. Had he made any promises about your future together?
Or maybe he is just over the relationship. I find it hard to believe that just one day out of the blue, things changed. Perhaps he forced himself to have stronger feelings and then one day he realized it just wasn't there for him and he was lying to himself. Or perhaps he met someone else. I think you are going to kill yourself trying to find an answer as to why. You just need to accept that it's over and try your hardest to move on. I don't believe a friendship will work when you still have feelings. Distance yourself from him, which I know it's hard when he's your neighbor but do your best. Only time heals those wounds.
Yes, you can stop forcing yourself on him and drop the guilt trip.
Instead of boring him with conversation about how not into you he is, cook a nice meal, then give him a nice QUIET full body massage.
No one likes to be nagged into having sex. Sex is best when it's spontaneous. If you are one of those who think that it is the man who should initiate sex, let me let you in on a little secret... they get tired of the theory that men are expected to make the first move.
An ounce of action is worth a ton of theory.