mental illness reason for sending claim late?

Its a first claim.

I thought learning problems are a type of mental health problem aren't they?

Such as learning how to fill in a form.

I agree. As we cannot give individual advice to members, I suggest you try your local CAB, as many of them have benefits advisers. If not, then try Community Legal Advice
who should be able to put you in touch with an adviser in your local area.

Nervy wrote:

I thought learning problems are a type of mental health problem aren't they?

Such as learning how to fill in a form.

I agree with you ..BUT the dwp seem to split them

I or my CPN could just say/write that i had difficulty learning how to fill in the form.

... and because of my learning difficulties.

right?

It would be best if you wrote, and your CPN wrote a letter of support, or you asked your CPN to write explaining why you were unable to submit your claim form in time.