Tons of good practical advice above. I can't think of much to add to that.
However, as someone said earlier on, you used to be able to do it, now you can't, so it sounds like you've developed some kind of block.
Firstly, that sort of thing seems to be relatively common in aviation. From what I've seen, the most likely things it happens with are landings, and nav. I managed to develop blocks with both of them when I was doing my PPL, so I'm a bit of an expert on this subject. <img src="wink.gif" border="0">
First, try and recognise what's happening in your head. With me, some little thing would go wrong; I wouldn't be quite sure where I was, and I'd over-react. I'd be convinced I was lost yet again, that I'd never get my PPL, and in no time at all I'd be too panicky to think clearly. I had to learn to orbit until I'd calmed down, then take a couple of deep breaths, and try and get my head sorted out before I sorted out the nav problem. The details may differ for you, but the general idea is probably the same.
Secondly, it may just possibly help to go over in detail what happened in all your navs, to try and find out what caused the block. It's probably some tiny little thing that you hardly noticed at the time, or were too busy to notice. Probably something that caused you to lose confidence in your own ability to navigate. Sometimes just finding out what it was can be all that's needed, though damned if I understand why <img src="confused.gif" border="0">
Above all, don't give up. Lots of us have had this problem, or similar ones. I'm sure you're not the first person to have to re-take their QXC twice. I was convinced for ages that I'd spent more hours than anyone in the history of aviation learning to land, and found out it definitely wasn't true. But if I was, so what? And if you are, so what? I still made it and so can you. Good luck!