No matter how hard, thorough, or long I practice, I can never seem to polish a piece so that when I perform it, there are no technical mistakes. What I mean by this, is flubs - missed notes, wrong notes, etc. Musically, I seem to be doing fine. But not technically. And this is very frustrating.
I am 17 years old and I have been playing piano for 7 years with a teacher. My most recent exam is a 7th grade piano exam.
My teacher says that I should practice slowly - play at a speed with a metronome where I make no mistakes. But no matter how slowly I go, I make a mistake somewhere. My teacher then says that if a make a mistake at the slow tempo I started at, then I should reduce the metronome speed. But I reduce the speed, and then make a mistake. Reduce the speed again, and then make a mistake again... ad infinitum. I feel so hopeless and I don't think it's helping at all. And being at school, busy, I often struggle to find enough time to
complete a slow practice session. They just take forever.
Worst of all, however, is that when I perform, I make mistakes that I've never made before. I also make mistakes in the easy parts of a piece, rather than the hard parts I've practiced.
Recently, I've been suspecting that maybe I'm not fully concentrating on my practice. Especially when I do slow practice, my mind tends to wander off, and I start daydreaming a little. Maybe this is why lose focus and concentration and am more prone to make mistakes. This sometimes happens when I perform at tempo as well, when my fingers and hands are flying, my eyes try to follow them and focus, I get dizzy and confused. If this may be the case, how can I fully focus and concentrate when I play?
This has been frustrating me for a long time...

Thank you.