You say that you can do the nav in the classroom, but it goes to pot in the air.
So you are capable of understanding what is required and what to do, but have a block putting it together in the aircraft.
As others have said, everyone has a block on some aspect of learning to fly. But eventually it clicks, unless you are a hopeless case. You have to weigh up how much you want to get that licence and decide for yourself whether to carry on until it clicks or give up. Obviously, if instructors were telling you that you are one of those hopeless cases, things would be different.
My stepfather started learning to fly at the age of 59. I have his logbook in front of me and it shows that he took 66 hours to solo and 120 hours to test (which he passed) at age 60. He found it tough to say the least, but he wanted it enough and persevered.
As for how to solve your particular nav "problems", there is a lot of advice on this thread on the techniques and I'm sure that much of it is good, but to be honest, you are not in the best position to judge which advice to follow and which to ignore.
Either trust your instructor or, if you have any doubts about the quality of the instruction you are getting, consider changing.