Sorry you're feeling low, dear...
I personally think that when I feel down the way you are describing, it's because I am repressing a lot of anger. Anger about being sick, getting old, and just not enjoying life. (oh, and those new wrinkles...ugh).
That's when it's great to go to bed, pull my covers up to my chin, and hibernate all day. Sometimes it feels like I'm taking a breather from the everyday humdrum when I do this.

I guess this is the part of MS when we have to really fight through it. Blech!