Maybe you could:
1. Summarise your conditions, symptoms + situation + what you need in a memo. Leave it at reception for your GP to read before your next appointment. Book a double/ long appointment and discuss how you are unable to work most of the time.
2. If she understands you could give her a draft letter of what you suggest is suitable evidence (referred to the descriptors)for her to review/ edit. GPs are entitled to charge ~ £15 for a "private letter" but it's probably worth it. Make sure you get a copy if it is to be sent directly to DWP.
3. If she hasn't already done so ask her to refer you to a consultant about your condition. Ask that health professional for a letter of support based on evidence.
4. Consider changing your doctor to one who has an interest in the condition and consequences.
5. Checkout
www.mssociety.org.uk if you haven't already done so. (Search for "ESA").
6. Try a Google search: esa "multiple sclerosis"
I found
www.msandme.co.uk/into-the-esa-support-group-i-go/ which might have something useful.
Hope something above helps,
Andy