I allowed myself to work on the piece only for 15 minutes a day and surprisingly it got a bit better just because I stopped obsessing about it so much but I still can´t play it faster than 120 BMP with evenness and there are sheets with tempo marked as 150 BPM... I am afraid I will NEVER be able to play it this fast in my life no matter how hard I practice...
You've explained exactly why kids seem to learn piano more easily than adults (though the reality is somewhat different......).
Kids don't obsess over insignificant things like trying to get a piece up to a certain metronome mark (assuming they actually own a metronome: I never did when I was a student), nor even about stuff like evenness once they try to play beyond a certain speed. And they're hardly likely to look up some whizz-kid's YT video and compare themselves to those prodigies who started lessons before they could walk.
What happens with kids is that once they play a piece to their teacher's satisfaction (no matter how slowly, probably not even completely fluently) they get moved on to something else, and they don't lose any sleep over that piece that's now behind them, never to resurface (unless they liked it so much that they wanted to have another go at practicing & playing it better by themselves in the future).
It helps if you can develop some of a kid's mentality with piano learning - be happy with what you can do with a teaching piece now, and only go back to the piece later on (I mean months or years, not days or weeks) only if you really like it and want to play it better when your skills have improved, and you'll find it surprisingly easy to pick it up again, and improve on what you can do with it now.
BTW, I retained some of my "kid's mentality" even now, when I'm learning difficult new pieces. There were a few pieces that I tried to learn when I returned to the piano in 2010 (Gaspard de la nuit
was my greatest challenge), and liked so much that I periodically returned to them over the course of months and years (playing them better and better each time as my technical skills improved - yes, I'm still improving), until I could play them to my satisfaction, so I could eventually perform them. And I'd keep on learning other pieces in the intervening period - all good stuff that helped to improve my skills in various ways.
Of course, if I got bored with those pieces at any time, I ditched them without a second thought - life is too short at my advanced age to bother with stuff that no longer interests me.....