So, 16 year old me has just finished secondary school (or has almost finished with exams). First of all you don't need to worry about your GCSE grades because you do really well in your GCSE exams. However, you're going to experience your first real 'failure' in a couple of years since you're not going to get the grades needed to get into your dream university, nor are you going to get the grade you deserved in your best subject despite getting A* in all of your mocks (of course you chose to mess up in the actual exam, the one that actually mattered). So you're going to feel pretty crap about it, then eventually you'll get more used to the idea of going to your insurance and you'll end up loving the environment it's in.
You're also going to feel incredibly isolated since pretty much all of your friends seem to be moving on and gradually ignoring you. But who knows, maybe things get better in time. You've met some nice people and started a hobby you've wanted to for a while, at least.