@ClemDanFango I'm so sorry you are feeling like this. I suspect lots of people feel like this so please know that you are not alone.
As I read your message I can honestly say I didn't get the feeling that you have low intelligence at all. It's not easy but the more we tell ourselves positive things, the more the brain changes (neuroplasticity) and starts to believe the positive things we tell ourselves. Perhaps write some positives down, even if you don't believe them right now, and read those every day! It's a small start to feeling positive about yourself.
I also wondered if some of your symptoms could be due to nerodivergance or be due to past trauma that hasn't been dealt with yet, even if you have been to counselling previously.
Your not being able to think clearly made me think of perimenopause /hormone issues. Or, perhaps an autoimmune condition. Some of what you say describes how I felt before I was diagnosed with that.
I don't want to overwhelm you at all, but just give some suggestions of things you might be able to look into.
Many people are low on vital vitamins such as vit b, magnesium and many other things. Perhaps a functional Dr/ health therapist might be able to do a blood test to check your levels.
I'm not medically trained so couldn't say whether it's physical, mental health or both issues, but whatever it is, the following could help the mind and body immensely in the meantime:
good sleep, eating fruit, vegetables and reducing processed foods, getting outside in the fresh air for a little walk, 5 minute stretching exercises (you tube), breathing exercises, talking positively to yourself, meditation (you tube), playing favourite music, talking to people...
I've also found listening to podcasts and educating myself about the above has really helped with my chronic condition, which affects my mind and body.
Most importantly, go to the Drs and try to see a 'good' GP who is understanding and prepared to run blood tests, make referrals and support you until you find an answer.
I know it's really easy to say and it's harder to do, but I hope the above suggestions will help you.
I know you can do this. You've been brave enough to write on here so there is a spark in you that wants to find out about why you're feeling like this. You have more strength than realise.
Wishing you the best of luck and I hope you find answers x