Well mainly what absolutely killed my motivation was my rejection from Cambridge but yeah I have exactly the same problems as you.
My STEP grades were only a result of hard work, I never got into any olympiad, I'm not actually very talented at maths at all; I certainly lack any creativity or any ability to understand anything remotely complicated in a reasonable length of time (it took me 2 f***ing days to actually understand the definition of a limit).
I'm not looking forward to the maths course at all. The only things I'm looking forward too are studying from my own maths books, maybe skip a few lectures to do so (I really don't care too much right now, but maybe that'll change as the workload increases and the course gets more interesting) and even more so being away from home and being able to be a lot of chess/tennis although I probably won't end up playing any tennis because it would require a certain degree of social skill.
I actually heard about that book and saw the pdf, will be studying that properly soon since Silman seems to really know how to teach. I'll also do that real board thing since it sounds fun and will be good prep going into OTB play in October.