You may be able to trade maths for something like logic or computer programming.

You may be able to get a disability special thing to do this.

Otherwise... Sounds like the remedial class is precisely for people in a similar position to you.

I tried to get into a similar class here last year, but the one here was also designed for people with no formal qualifications and I had formal qualifications in things that didn't involve maths.

Don't freak out about the test... It is diagnostic... And you already know that you won't do so well on it. Khan academy might be helpful for you... But feeling like you have to get through it all on your own... Would probably be overwhelming.

I hope you get good teachers. They can make a huge difference.