Then you study too much inpractical variations. Its okay to learn some of the main line moves, but you have to play, then analyze, then play, then analyze, and so on.
So you got a feeling how much opening knowledge your opposition has and you can find out what your mistakes are and how good you profit from your opponents mistakes.
Sometimes you have to learn as well, e. g. when your oppoent outplayed you in a sharp kings gambit line.
I'm a beginner player, so I study some openings but as I play in the beginner levels, when I want to practice it, my opponent doesn't play the lines he supposed to play, like he doesn't play the best moves in that line.... so I get mixed up and couldn't practice the opening line I studied.
How should I practice it?