MG or CFIDS or something else????

Hi - I am hoping someone out there can help me!

For several years now, I have had some symptoms that seem to be stumping my doctors.
The most significant ones I experience are:

1. Severe physical and cognitive fatigue after exertion (eg after a 30 minute run, after carrying moderately heavy groceries up the stairs, etc...). It takes me a good 24 hours to recover, and even then, the affected muscles feel 'bruised' for days.
2. Constant fatigue - I could sleep 20 hours a day if I didn't have to get up.
3. Dizziness upon rising/standing - my bp is usually around 90/65
4. decreased mental alertness - varies throughout the day, worse in morning
5. low mood (this improves when my cognition improves, which is rarely)
6. constant pressure at the base/back of my head, pressure in upper chest
7. left eye droops intermittently
8. neck and back pain - chronic

Does this sound like MG at all? I am also wondering about CFIDS/ME? One other possibility someone suggested to me was a Glycogen storage disease? I am willing to accept that some symptoms indicate possible depression, but the inability to exercise or recover afterwards doesn't seem to fit with depression. I don't want to start down the path of anti-depressants until I am sure that no other dx fits. BTW - I am on thyroid meds, but these symptoms are still there even when I am well-dosed.

Thanks!!