I was a Head of Department but within a year have become 0.6 mainstream teacher and am in the midst of applying for early retirement. Fatigue, cognitive stuff and pain are my biggest issues.
Early retirement through ill health is complicated though and I would advise that you get your union on side for advice (ASCL I think?) And if you aren’t already a member, join now, you do not have to disclose that you have MS, yet). The government are keen that you stay in work and as you probably know, schools have to make reasonable adjustments, for me this means that I have less hours (less pay), a bed! and less duties. I have ‘Access to Work’ support who provide taxis to and from work (I donate a third of the cost.
The LA have given me an Occupational Health consultant and he is instrumental to my application (he writes in the first section on the application forms). Phone Teacher Pensions and they will send you the forms. And seek independent financial advice.
Because you are a Headteacher, I doubt that you will have to become mainstream or middle management (as i have had to do) before applying, but you will need some absence from work or other indicators to prove that you are incapable of carrying out your job. There are two tiers of teacher pensions, so you really do need to accept advice along the way.
I do hope that you are successful and remember to try to be kind to yourself.
kindest thoughts Ali