what exactly is the problem? Divide your big task - the navigation - into smaller chunks - 1) keeping heading, 2) reading chart, 3) calculations etc. There's no point in trying to do all together if you struggle at these things individually
have you done instrument flying yet? If not, do it! Make sure you can keep heading, altitude and speed without visual reference. Once you get those sorted, go back to nav - at this point you should be able to keep your heading, so from there on you have to focus on map reading and dead reckoning.
Anyway 80 hours is a lot, at that point I would start to wonder would you not be better off with a new instructor?