This is it exactly for me, I feel exactly the same as you do. I had a promising career as a Clinical Psychologist, I was going to apply to start my doctorate this December.
I had 2 accidents, one in December last year in which I'm pretty sure I sustained a concussion, then a minor car accident a month later, after which I started experiencing symptoms.

I returned to work in July for 2 half days and it was just overwhelming and bizarre. I looked over some emails I had written before and some documents I had put together before the accidents and it was like they had been written by someone else.
I can't write to that standard of English anymore or get across complex ideas.

But, like you said, the thing that scares me the most is I feel like a different person. I, too, was very active both mentally and physically. I loved being smart and it was a big part of who I was, gave me a sense of validation, self-esteem and confidence.
I was a very strong, social and independent person and I'm just not anymore.

Now I can't use big words anymore as I'm no longer sure of their correct use and often get them wrong, so to avoid embarrassment I speak a lot more simply.
I don't get jokes, either at all or quickly enough to not look really stupid in social situations (this is leading me to avoid some social situations).

I have no confidence in myself anymore and I have no idea what I can and can't do in terms of my cognitive abilities.
Whenever they are 'tested' by a challenging situation, then I seem to fail pretty quick. I spend most of my days doing light tasks which tax neither my brain nor my body and sometimes I feel 'normal'. I don't feel myself, but I feel like I don't have issues.
However, as soon as I am mentally challenged then this all falls apart and I realise I'm still broken.

I think, like you, most of all, I was happy and a very cheerful person. I don't feel like that anymore. I realised I rarely smile or take much pleasure in doing anything and it's taken me some time to accept that I am depressed.

I'm 7 months in and when I first came on the forum I felt a mixture of relief at finding some answers to questions I had and sadness at the answers themselves. It was hard to read about damage being permanent, but obviously made sense when I thought about it (I studied the brain somewhat, I should have known this!). I'm also awware that it's only some stuff that can be rerouted so to speak, so there are deficits I have that I will always have.

I also felt really down initially at the prospect of never being back to who I was before the accident and I think I've just started the long road of coming to terms with that.

I try not to be negative about it, but instead I feel I'm being realistic. I will never be a Clinical Psychologist now. I may never have a high status/complex job (this was very important to me). I may never improve much beyond my current situation.
BUT, I keep telling myself, like you said, that it could be so much worse and I can still live a very full life.
I keep trying to think of all the things I CAN do and not everything I can't.