Playing the part you already know feels great and teaches you nothing.

Rehearsing what you have already mastered is pleasant precisely because nothing is at risk. It produces the sound of competence and no new competence, since learning happens at the edge where you are still unreliable. Comfortable practice is really a small performance for an audience of one. The hour counts on your tally and nowhere else.

Split your next session: a few minutes of the part you love, then the rest on the bar you keep fudging. Work slowly enough to play it correctly, then speed up. If a session was entirely enjoyable, suspect you spent it in the easy place.