What should I do, after I have given a guy so many chances and he yet keeps messing up?

Try talking to him telling him that you're unhappy with him and if he can't stop hurting you, that you will leave. There's no true way to drift away slowely that I know of. If you are unhappy you have to let him know, it's going to hurt him either way and there's no way around that. Tell him in a calm but serious tone that way he knows you are serious. You still can be freinds with him, hanging out and stuff, but the relatiionship has to come to an end (no more bf/gf). Mainly just talk to him about his behavior cause if you don't, he will continue.