Post Diagnostic Depression, eh?
I know this is a serious topic. Having a chronic illness is not fun and everyone has a different level of ability to cope with it. Yeah, I completely think that seeing a psychologist or psychiatrist - one who specializes in chronic illness - is a great idea. Why? Because they can give you coping skills, unique to you or not, that help tremendously with getting through hard times. They can talk about what triggers your sadness, what makes it better. You can say things to them that you can't sometimes say to others, thereby making it a very honest and useful discussion. Though I find any appointments to be exhausting, so be wary of that. And you don't have to see someone on a weekly basis or anything. Even every few months might work.
I know what makes me ok on a daily basis and work at keeping things that way. Music is a big deal for me, it usually pulls me out of a funk. Or learning something new - love the History Channel. Fish truly is a great mood food, as are others.
But it's about what makes YOU feel better that counts. I know you've been trying to get diagnosed for way too long and you might think that getting that diagnosis means that you can suddenly "adjust" to MG. Yeah, right. It's been almost ten years since diagnosis for me and I still catch myself in complete denial. It's so hard to adjust to the "finality" of it all.
Set yourself up for success. Rethink how you WANT to live your best life. And let it stew for awhile. It's really an art form to be able to carve out a little happy place for yourself, in spite of MG.

Annie