If a musician makes a mistake, or simply cannot play a passage...what is the reason?
I have heard many answers to this question but the only correct answer is: "You are going too fast".
If you cannot play a particular song or exercise you are practicing, then it is because you are going too fast, so, to fix the problem...slow down!!
Take on a small portion of a song at a time. Maybe 2 to 4 measures.
Say the names of the notes contained in those measures out loud, in rhythm. Then play that small section one-note-at-a-time. As slow as necessary to hear each note sound.
Repeat over and over, quickening the tempo every time you repeat.
When you make a mistake, slow down and start again.
Before you know it you will have that part memorized, mastered and sounding good. You'll be ready to take on the next two measures in the same manner.
In no time at all you will have the entire song learned.
If you can't play what you are trying to practice and learn...slow down!
Once you can play it slow, then off you go!!
It will be easy to play that part faster now.