I wouldn't worry too much. When I was in your position, I had the same problem, I had to choose between maths and physics and I didn't think I would be able to write anything for either of them. I decided not to choose and go for both of them, but when I actually got down to writing it, I realised that it is a lot easier than I expected. The first draft will be awful, but after sending it to a few people for feedback and rewriting it each time, it gets much better.