Sorry to hear about your diagnosis.
One of the (few) consolations of MS is it doesn’t come with a huge list of dos and don’ts. Everybody’s different, so you just have to learn to interpret what your body’s telling you about what feels OK, and what doesn’t.
Even if you get it wrong sometimes, it’s unlikely you’ll be doing yourself any permanent harm. You may get an unpleasant but temporary worsening of symptoms, but these are not a sign you’re making the illness progress any faster. You just need to get used to what’s fine for you, and what isn’t, and the only way you find out is by trial and error.
Some people don’t get the heat thing at all; I never really have. However, if you know you’re affected, you just don’t have the bath/shower so hot, and you get used to what temperature works OK for you. As I say, if you misjudge it while you’re still experimenting, it’s not catastrophic - you won’t have caused irreparable harm. You’ll just learn the hard way that your body didn’t like that so very much, and perhaps not to do it the same way next time.
I think of it a bit like having flu’ - except, unfortunately, it’s flu’ that won’t go away. But just as with flu’, there’s no absolute ban on things you can do, but you probably won’t be able to cope with everything you normally would - the same with MS. It’s commonsense, rather than rigid rules.
As you get more used to it, you’ll know if your body is telling you it doesn’t like something. Even then, there might be valid times you choose to ignore it. For example, sometimes I do stuff I know is going to make me feel ropey later/tomorrow.
But if it’s important stuff I really want to do - like a special day out, or meeting a friend I haven’t seen for ages, I might decide feeling grotty afterwards is worth it, and deliberately keep the next day free, in case I need to stay in bed.
Does this help at all?
Don’t get too bogged down with rules!