I've been screwed over by Christians so there is no way that I could ever become a Christian.

I could never become a Christian because I know a Christian who treated me badly.

Actually this is a pretty common reason for people to quit many things. A baseball player is convicted of a hit-and-run so you stop going to any more sporting events. A doctor is convicted of sexual assault so you rant and rave about the evil that's endemic in the medical profession. Woody Allen has sex with and then marries one of his children so you stop going to the movies. A friend breaks a promise so you tell people how all friendships are rooted in hypocrisy. Your child butchers a piece by Mozart on the piano so you claim that Mozart was a fraud. Oh wait! You actually don’t do that. In fact, that kind of reaction is reserved only for excusing your rejection of Jesus the Christ. Ever wonder why there's that kind of inconsistency in your life?

Look, the Bible tells us that "the heart is desperately wicked." Not just my heart but yours as well. While becoming a Christian should, and in the vast majority of case does change our behaviour for the better, we are and will remain consistently drawn to selfish, self-centred, unloving behaviours toward those who are closest to us. But that isn’t Jesus' fault. The number of lives that He has radically changed for the better are legion and indisputable. Jesus can do that for you. Don’t blame the composer for the student’s poor performance.
Jeremiah 2:5 This is what the Lord says: "What fault did your fathers find in Me, that they strayed so far from me?"

No body is so good that they don’t need forgiveness from God. No body is so bad that they cannot receive forgiveness from God upon repentance.