As for delaying A levels medically, I probably could. The problem is, this whole year has had a silver lining that's kept me going throughout: My school year had an abnormal intake of lots of brilliant people that are amazing, supportive friends. In particular, there's someone who is like me if I hadn't had the concussion. While it stings to look at the difference in where we are, they have all been immensely supportive. It also makes me feel physically safer - when I'm surrounded by friends that I know care about me I can actually relax. I don't know what I'd do without them.
Every year my school's intake gets worse as it becomes larger and more mainstream. It doesn't have an ability to properly discriminate due to the laws against new grammar schools. I doubt I would find this level of support if I waited a year, and that support is what has kept me going. Plus, I improved significantly last summer break; I can hopefully have a similar improvement in the upcoming 6 weeks.
You mention the need to grieve for what I've lost. You're entirely correct. This wouldn't be anywhere near as bad if I didn't know what it was like to have things come easily and the certainty of a bright future. I need to get over it somehow. I operate in strategic long-term objectives (This year I will achieve x, y, z) and I began this year deciding that no matter what, I would achieve my prior objectives. That's obviously been a complete failure. Changing my objectives is a difficult thing to stomach. It used to be that I had high expectations of myself and I would consistently achieve them. If I had problems, I would simply reallocate some time from casual programming/gaming/etc to that objective and then achieve it. I have no more time to reallocate.
Thank you. Your comment has prompted me to stop avoiding some difficult questions and work them out.