OP, I’ve been where you are and I know how terrifying it is. My worst fears came true, but I hope it helps you to know that ten years later I manage my symptoms by triathlon training. Not how I expected my life would be when I was first diagnosed!
You seem to be quite concerned with looking for causes with which to blame yourself (apologies if I’m wrong, that’s just the impression I’m getting). The cause isn’t known, there are factors like Vit D etc which appear to be linked, but nothing has been proven to have a causal, rather than correlating link. Stop looking for ways to blame yourself, it won’t help a bit.
What you can do if it is MS, is put your thoughts into managing symptoms - stretching and strength work in short daily doses are very effective, and if you have fatigue as a symptom, ask for an Occupational Therapist referral as soon as you’re diagnosed. The OTs have changed my life. An MS physio referral early on is a good idea too, get on their books so that if you get a problem later on you can be seen quickly and start the recovery process fast.
It’s a massive adjustment, and the mourning for your old life and expectations can be hard. But I don’t just “cope”, I enjoy my life. It may not feel like much comfort right now, but you can too. Good luck, I know it’s scary.