You talk about brain damage and head injuries and having had the wrong medicine. I can't tell if this is what the doctors concluded or what you suspect.

Your brain is not a static thing. It can be thrown out of balance and behave defectively. But the balance can also be restored, and things will be fine then.
The brain can also go into a feedback loop, both negative and positive.

Many things you say I can relate to. I am 32 now and only a few years ago I managed to get into university and perform at my natural level. I failed very simple math texts in high school, but now I study physics at a top50 university. I have to do complex math. I am taking an MRI/NMR course now.

I have been in an MRI machine myself. It is a scary and intimidating machine and it can create anxiety. But the energy levels involved are very very low. CT is x ray. CT shoots high energy particles through your body.
MRI measures very small energy differences between magnetic atoms in your body by shooting radio waves at exactly that energy difference. The energy difference would be immeasurably small in a weak magnetic field, so a strong magnetic field is used.
X ray/CT has like a billion times more energy per photon.
MRI makes molecules rotate. It just tickles a molecule. It cannot even make molecular bonds vibrate. Too low energy. X ray knocks electrons out of the molecules, breaking the whole bond and destroying the molecule.

Don't become hypochondriac. Yes, a neurologist will have a certain way of thinking and so will a psychiatrist.

To me, most things that you say seem to be anxiety and hypochondriac, except for the headaches.
Failing your math text or not being able to get into university, that is frustrating, I know. It took me 12 years. You can still fail for three more years and graduate before I even got in, if we compare times.
If I believed I could do it, I would have gotten in earlier. If I had sought out help rather than being stubborn and refusing help, I would have gotten in earlier. Now that I am in, I thrive academically. Something nothing in my past suggested I was actually capable of. Only the arrogant minority of my brain actually believed I belonged there.
Not saying the same is true for you, but just because you have been failing doesn't mean you can't do it.

Now you can be honestly anxious and frustrated for many reasons and of course you have a mental health history. But it doesn't seem things are really as seriously wrong as maybe you make it out to be. You talk about very serious things, but reading this post and a very shallow glance of your post history, it seems that right now in your life, there is more the fear and anxiety about something being neurologically wrong with you.

I used to have panic attacks. I knew 100% that it was irrational. And getting them was not befitting of my character. I am very macho, stoic, outwardly though, fearless. Still, they happened. I don't have them any more.