I've experienced the same thing when dealing with people. I think I'm so nice, but then suddenly this monster (as you mentioned) comes out of nowhere. I think that's the human condition--we all have a monster of selfishness inside, and sometimes God allows trials to help us see our weaknesses. Think of Peter in the Bible. He knew he wouldn't deny Jesus, but when he let his self-righteousness take control, he could stoop to any evil. Sometimes we need to see how awful our sinful hearts are before we can realize our need of Jesus. Peter had to learn the hard way that we can't do anything without Jesus (John 15).
But with Jesus, we can overcome even our "monsters" inside! When we're connected to Him, He helps us to gain victory. But I would say don't expect to become perfect overnight. Jesus takes us by steps; He doesn't teleport us. We can be perfect in Him moment by moment, and each stage of perfection in Him leads upward as we're transformed into His image from "glory to glory" (2 Cor. 3:18). And also, just because you feel like a monster inside, it doesn't mean you have to act on those feelings. If you have repented and asked forgiveness from old mistakes, then believe that Jesus will be transforming you from the inside out, even if you don't feel it. Don't be discouraged by your failures. As long as you look to Jesus for help instead of yourself, He'll take you forward.