That's very frustrating, right? You practice for hours and hours but feel like you are spinning your wheels.
Here's the thing: when you practice, make sure you do all the separate voices, all 2 part combinations and all 3 part combinations in 50 percent slower tempo at least 3 times in a row correctly (with correct fingering, articulation etc).
Please insist on the current step before going on to the next one. That's why it's too difficult for you - you probably take the next step too soon.
[Thanks to Carolina]
Transposing Sequence in C Minor: i-iv64-ii42-i