I don't think orthodox/Western medicine recognises "damp" as an aggravating factor, but you'll nevertheless read many accounts from MSers (I'm one of them) who swear they feel worse in this kind of weather!
And my acupuncturist told me traditional Chinese medicine doesn't recognise MS at all (because it was not yet discovered and named when traditional medicine first developed). So in his alternative interpretation, according to Chinese medicine, ALL my symptoms were due to "damp". (Don't take this too literally; I'm not a complete crackpot, and I do believe MS exists - my acupuncturist just had a different way of looking at it).
I think the expression "under the weather" didn't originate for nothing; it does aggravate a lot of things.
I am nearly a year ahead of you, in terms of diagnosis, so have had a little more time to get used to the idea. Like you, I was worried (and still am, to an extent) that my decline would be fast. I can only say that it hasn't been the case so far, although life certainly isn't getting any easier, or symptoms any less - but I didn't expect them to.
I have learnt it's important - as far as possible - to take things as they come, and try not to keep worrying about how bad things might get, or how soon - because they also might not. If you're not careful, you can spoil what is good, by obsessing about things that haven't happened yet, and perhaps never will.
I would strongly suggest you do choose a relative or friend to confide in, even if you don't open up to all of them, because you'e making it harder on yourself if, in addition to all the problems MS brings, you've got the added pressure of trying to appear happy and jokey. What are friends for, if you've got to put on an act for them, and not tell them how you'e really feeling?
If you absolutely couldn't tell a friend, would you consider talking to a counsellor? You don't owe them anything, because it's their job, so you haven't got to put on a happy face for them. At least you would have someone to talk to about how you really feel, and you wouldn't have to worry about how they're going to take it.
On the DMD side, can't advise, as I'm not on any. But fingers crossed the scan will show the Avonex is working well for you. I think it's normal they would want to check, and also normal that they'd suggest a change, IF it hasn't been working.
Oh, one more thing - disability is NOT closely associated with the number of lesions, so even if your scan does find some new ones, it's not an accurate predictor of how disabled you're going to be, or how quickly. The brain is remarkably good at rerouting, to minimise the effects of damage, so it's possible to have loads of lesions, but still be functioning pretty well.