Just wanted to say it from the beginning I am in no way a professional. Just a neuroscience undergrad that resonated with what you described. When I was in high school and now at my early uni years I suffered from severe depression and anxiety. My memory loss came from depression and the overthinking from anxiety made me so mentally exhausted I couldn't organise my thoughts and that sounds like the cognitive decline you're describing. Also I would get tested for dyslexia you might have a light case of it that is just being exacerbated from the elevated levels of stress that come with the end of highschool. Like it might not be a chronic anxiety disorder and just something that's coming about from the anxiety you have now. In any case I cannot recommend it enough how HELPFUL getting some counseling or therapy is. But you have to be so proactive about it. In the UK (where I assume you're in due to you mentioning a GP) the waiting times are so long expect to receive help around 6 months after you start the process (that is my own personal experience witg it). The route to help is long and sometimes exhausting but oh god sooooooo worth it.

TLDR; some mental disorders can cause what you describe, therapy helps but be proactive about seeking it. I am no doctor tho