I am with man who is always giving but all i ever do is take?

I have been wih this man for almost two years now. He has given up his whole life for me and I keep screwing things up, he is the best thing that has ever happened to me and if it weren't for him stopping me I would have killed myself a long time ago so why am I still so selfish. I hate it but don't even realize I am doing it until it is pointed out to me. Please help if you can , I need advice , I think he's gonig to leave me.