I told him I have no feelings for him when i like him alot?

It happens all the time. When I am in a relationship, I am always awkward and cold to him. I don't know why, I am actually kind of shy. And when I am trying to hide the fact that I'm shy, I say things like "just go away" after that, I start to blame myself. How can I be true to him? we're not together anymore and I don't want things like this to happen in my next relationship again.