Are you working mainly in the U.K.? You should be protected by the Equality Act 2010 which says that an employer must make reasonable adjustments for an employee who has a chronic condition that affects their ability to work. It’s clear that you should still be able to work and ‘reasonable’ is hard to define, but HR should take note of the Equality Act. It applies to all whose work is based in the U.K. regardless of their country of origin.
The Disability Discrimination Act might also be worth a look. Unfortunately, T1 is classed as a disability even if most of us don’t feel disabled - except when T1 tears up and hits us in the face with a wet fish, of course.
The other thing you might consider would be to ask for health-based extended leave to give you space to sort out the parts of T1 that are plaguing you. Have you worked with the firm long enough to qualify for sick leave?