When I got aware of some ability, I couldn't do it that well anymore
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.