It’s a known fact that, statistically, people with one autoimmune disease are at higher risk of another, so please don’t beat yourself up thinking it’s anything you did. You were probably just unlucky when the genes got handed out - nothing any of us can do about that. I do not believe EVERY disease is preventable with lifestyle changes, although there seems to be a huge fashion at the moment, for thinking that - perhaps out of a misguided belief that if you can reduce the risk of heart disease or diabetes, the same must be true of everything. But in that case, sporty vegetarians would never get sick, and we know that isn’t the case.
Some risks are in your genes. Others might be environmental, but things you have little or no control over, such as what you’re breathing in, NOT what you eat.
Although most of us can take steps to lead healthier lives, I do not believe we can plan never to get another disease, as too many things are outside our control, and could be described as luck, or fate, or whatever you choose to call it.
I think we have to accept that sometimes, sh*t happens - and sometimes it happens more than once. It isn’t always something we could have dodged if we’d tried harder. So if you already have a healthy diet, I don’t think it’s very likely you are doing anything “wrong” that’s causing all the diseases.
One something does strike me, though: I think temporal arteritis/GCA were differential diagnoses that came up for me when I was being investigated for MS, as I gather the presentation can be very similar. I suppose the MS Dx is not in question, and it could have been one of these other things all the time?