If you can't work a sedentary job consistently 8 hours a day, 5 days a week, then you would qualify for Social Security disability. "Sedentary" is the lightest job activity level. The other two are "light" and "medium".
You may be able to get disability for the heavier types of work if you are 55+, and have only a high school education or less. The reason is because they feel you're too old to be retrained into lighter work.
The reason to not work part time or seasonal instead of going on disability is because your amount of disability is based on how much you earned in the past five years. So if you're earning a part time wage, your disability income will be tiny.
You can work part time while you're on disability, as long as you keep it under the Substantial Gainful Activity amount. However, while you're waiting for disability, it's best if you're not working at all. Their thought process is, if you're able to work a little, why can't you work full time? So it's better to show that you can't work at all.
However, I have heard of a few people who did get approved while working part time - you need to keep it under the Substantial Gainful Activity amount (last time I checked, about $1000 a month) or they will deny you for sure.
You can use getting fired from jobs as proof of "failed work attempts".
A good number of people with ME/CFS or FM have had luck using a service such as Allsup to prepare their disability claim. They know the right things to put in your claim so you get approved sooner. Most lawyers won't take you on until you've been denied, but that seems to be changing with pressure from places like Allsup.