I totally relate to this too. I’ve been saying this exact thing to my sister. Cancer is horrible, but I don’t think I’ve met anyone who doesn’t have empathy and concern for a cancer patient. Never have I heard someone’s response as “I know so and so who had cancer for 20 years….. its no big deal. Or, oh you have cancer? Well you look good!” That would be considered heartless, but that is what we hear about MS all the time. Somebody always knows someone else with MS like it’s just some common ailment or nuisance and we are all the same.

I often hear, oh I know so and so who has MS and they’re doing fine

It's very annoying/disconcerting, when someone has "advice", often unsolicited. I think people feel anxious when someone has a medical issue and they want to "do" something.. It's extremely frustrating and it's ok for you to be annoyed. Even if it's coming from a "a good place". Maybe: Try to gently tell them you appreciate their concern, and/but that you don't feel like discussing it right now.