When I got aware of some ability, I couldn't do it that well anymore Every time I got aware of something I could do well, it got a little bit worse.

Examples:


I could immitate a lot of accents or people, but when I thought about it "Hey, I am very good at immitating something", I couldn't do it that well anymore. 
I could sing a tone nearly the same it was played to me, but when I thought about it ( that I could do it well ), I couldn't do it that well anymore.
I was able to have a good conversation with nearly everyone, but when I thought about it, that I am good at it, ......
and so on....


I know that I wasn't thinking I could do those better than I could in reality! ( because than it would be logically why it seemed to me that it got worse )

And when I wasn't thinking, that I am good at for example immitating, I was able to do it that well again.

My question is: Why does this happen?

I mean, either I am good at it or not right?
It makes sense that you should either be good at something or not, but in fact our abilities vary a lot under different conditions. 
It sounds like what you are experiencing is loss of automaticity. There are things we do best without the interference of the conscious mind, like playing sports or driving or any skill that we have practiced a lot of times. When we pay attention to what we are doing, we may not be able to do the action as quickly or as well.
It might also have something to do with "choking" under pressure to do things well. There was no pressure before you decided you are good at it, then it became harder to do it well after that.
There's an older paper that explores these ideas. Shiffrin, R.M., &amp; Schneider, W. (1977). Controlled and automatic human information processing. II. Perceptual learning, automatic attending and a general theory. Psychological Review, 84(2), 127-190.