You aren't being possessive but if the two of you were just hanging out in the beginning then those messages shouldn't even matter! Oh and guys say the stupidest things EVER, so try not to read into his little "you should have been more possessive" comment. Obviously he has no idea what he is talking about. Plus, you need to learn to bury the hatchet on some things. You don't need to revisit past problems constantly in your relationship, it will only continue to hurt things in the future.
I'm not good at beating around the bush or sugar-coating things. My responses are never personal vendetta's on your emotional state. It is what it is, whether you choose to accept it or not. <3