Sounds like the maths tripos:

At A-level I would just look at a question and know what to do. This was nice.

In the first year, I would look at a question, not know what to do, try things that didn't work for hours, then go away, and by some miracle usually work it out when I came back to it the next day.

In the second year, I would look at a question, not know what to do, give up, come back to it the next day (as this was obviously the best way of solving it, right?) remain stuck, and never get the question out.

By third year I realised that no matter how miserable and depressing it was, you had to keep trying all these pointless dead ends for hours and hours - that was where far more of the thinking happened then following a way that actually works... But it really hurt, going back to that hands-on stupidity...

Hmmm. You know, I think that happened to me too, only one year behind. In first and second year maths I did pretty well, but in the third year I started to lose it and ended up with "only" a 2.1 (yes, I know plenty of people would have been delighted with that but it was a disappointment since I'd had firsts up until then). If only I'd started to lose it a little sooner, perhaps I would have had a chance to recover from the lapse before doing my finals...