Hi! A few questions:
1. Do you plan to study with the OU full-time (3 years)? If so, how are your existing maths skills? Have you tried the "are you ready" quizzes for MST124/MST125?
2. When does your access course finish? Would it overlap with the degree at all?
3. Academic writing is an important skill for any degree, even for a less essay-based subject like maths. The fact that your access course focuses on it would seem like a good reason to continue with it, but obviously your mental health has to come first.
4. Is it possible that you have dyslexia or something similar? If you do, and you can get an official diagnosis, you can often get certain accommodations in your course(s). Definitely worth checking.