Why did you agree to advance your relationship from "just friends" to romantic partners if you knew him to be the way he is which is negative, sulky and non-confident in much of everything? Did you think he would suddenly be a happy camper with a good and healthy personality?
When you break up with him, which you should do... tell him outright its because he's negative and whiney and that you've known him long enough to know that he's not changing anytime soon.
Then don't continue to be his friend because you both need to go zero contact and forget one another so that he can hopefully learn something from what you told him and go on to meet someone to be with in a more positive manner then what being with you has enabled him to be. And.. so that you can find someone else to be with in a more happy and solid way then his emotional whining post.
Good luck. Don't stay in his life ... It will do neither of you any good.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion