Most Helpful Girl
I had the SAME problem with writing. I've been writing school essays so much and then I took a creative writing class and I was like shit I can't be creative anymore because everything feels like a school essay. Honestly I think you just have to do it on your free time and pick your own idea and just go with it. I still lose my voice sometimes when I write. It's terrible, but sometimes I read things I wrote before and it kind of puts me back in my own mind. Look at some of your art from before those hard classes and remember how you felt then and try to put yourself back in time in that position.
Most Helpful Guy
That actually is very hard - make sure you avoid those sort of things as much as possible - just pass, duck & dive - so you can preserve your liking - which is the only thing that will actually drive you.
To improve on the damage done - step sideways, find a slightly different expression - so you can find the original interest back, then re-apply it forgetting about the exam experience/isolating it (as idiocy or whatever it was)