I agree with this. It's a massively tough market at the moment for employees.
I posted an admin job and had over 200 applications in 48 hours. I'm always going to choose the person that's most qualified for the job, so I'm afraid to say, you wouldn't make the first cut.
Have you tried temp agencies, even to do factory work? It's been such a long time that I think you might just have to accept anything to start building experience and confidence.
I would definitely push for some sort of diagnosis though because the struggle with maths and English GCSEs would definitely suggest some form of learning difficulty like dyscalculia or dyslexia (or both).
At least then you might understand why you struggle compared to others, and it might open up pathways for support or guidance to help you find a path.
You write well enough on here to show that you're capable of corresponding and communicating with people.
As a PP said, if you were to look at yourself, what would you say you're good at? If you were to ask your closest family and friends the same question, what do you think they would say?
Can you make an appointment at the job centre to discuss things? Or the Citizens Advice Bureau? They might be able to give you some other ideas.