SOS in Bach

[mozbee]

Hello, I have been playing the piano for almost 3 years.
This year, i am practicing Czerny op 299, and Bach invention/sinfonia, Beethoven sonatas.
1.Bach's pieces are my biggest problems, and i could not further play Bach's anymore, are there any tips that it might help?
2.In most of the pieces that i have been playing, i always have a mistake(like wrong notes, stopping), and i could not perfect the songs entirely.. I have almost finished Czerny op 299. but i couldn't play the given tempo of the pieces, help please~

[indianajo]

You start with each hand alone, slow enough that you make absolutely no mistakes.  If that speed is 4 bpm (quarter note = 15 seconds) , so be it.  If you make any mistake twice, write in pencil the note name above it.  Don't speed up any until you have played the entire piece one hand alone without mistakes, twice.  Then only speed up 10%. 
Eventually, your lower brain knows the movements well enough you don't need to look at the music.  Then you can put the hands together, slowly again.