3 reasons people can't play polyphonic music

1. The texture is too thick. If you can't play SATB together, play STB. If you can't play STB, play TB. If you can't play TB, play the bass alone etc.
2. The tempo is too fast. If you can't play at the concert speed, play at the half speed.
3. Insufficient number of repetitions in each step. If you can't play each combination and fragment at least 3 times in a row correctly (5, if you're aiming for the moon; 10, if you're reaching for the stars), you are not ready to go on to the next step. No excuses. Period.

Look at your playing. If you are stuck, aren't these the reasons why you may be spinning your wheels?

You can change this.

Sight-reading:
Adagio (p. 7) from the Trio Sonata No. 1 by J.S. Bach.