I have it following radiotherapy to treat cancer a few years ago. At first I thought it was just post-treatment tiredness and would improve, but it just didn't. I eventually had to give up work because I was spending work days and much of my commute time shaking with fatigue and non-work days and evenings resting in bed to recover. My job was very challenging and demanding and DH and I had no quality of life together, so it couldn't go on.
I'm incredibly lucky in that I have a very supportive husband and adult children, but the restrictions of CFS are still upsetting and frustrating. I never bothered to get formal treatment because: 1) my realisation was just before Covid so it was soon difficult to access treatment; 2) I'm actually an OT so I've been able to manage it fairly well, make and follow my own energy management plans etc and 3) My local fatigue clinic is some distance away and I knew that each journey/appointment there wouldn't tell me anything I didn't know and would set me back so much that I'd spend all time resting to recover until the next appointment.
I'm now doing pretty well most of the time, can be busy for several hours a day, including long walks, sleep fairly well and when I overdo things a little I can usually recover overnight. I know my limits and only really struggle if I've been ill and had to be inactive for a week or two. I find that forced inactivity causes a relapse just as much as doing far too much does, so I'm careful not to rest for too long between planned activities. Having to manage my life and activities within such narrow parameters drives me mad at times, but it does allow me to be relatively consistently active and busy.
When planning your day it can help to think of your energy as available 'blocks of time', so 5 x 20 minutes a day, 8 x 10 minutes, or whatever your levels are, with planned rests between using each block. Also remember that things like personal care, eating and so on, need to be included in the energy you expend. It's really worth putting a basic daily timetable together, which you can then tweak until it reflects what you're able to do comfortably without causing post exertional malaise. Then you have a baseline to work from if you want to try to slightly increase what you can do. Also, knowing what you are able to manage consistently is less frustrating than forever overdoing things on good days, followed by several days of post exertional malaise.
Puzzles, online quizzes, informal studying or whatever mental activity works for you, helps with brain fog without putting on additional pressure or (if you stop when you begin to tire) worsening your fatigue.
Research shows that most people do recover to at least some extent and that pacing and energy management are the most helpful physical interventions to aid recovery. It's still shit, but in my case I'd be dead without the radiotherapy (cancer had spread to my lymph nodes), so I shouldn't really complain.