I did too much this weekend and now I'm paying the price, still have chores to do and obligations to fulfil but can barely get up off the sofa - what do you do at the point when you realise you've overdone it?
Normally, triage extensively to the things that absolutely have to get done and nothing else, and then work on figuring out a) how to get the rest of it dealt with promptly and communicate that and b) how to avoid that particular thing coming up again.
Stuff I've done that helps on the latter, which may or may not be relevant to you
(I live by myself, so as long as both I and the cat have basic necessities, everything else can usually wait a few days if it has to, even if that's not ideal. I have moderately decent sick time from work, and I have a lot of autonomy on what I work on when, so I can do 'can't brain today' tasks when needed. The previous sentence was key to my last job hunt.)
1) I use a pedometer app on my phone.Not for "Yay, I did X steps!" but so that I know if I went over my usual significantly that I'll probably be low on spoons for a couple of days. This helps me get ahead of the "Argh, can't get out of bed, have things to do" problem.
2) Limit how much I schedule outside of core obligations pretty strictly. Core stuff is work, swimming (consistent exercise helps with some of the health stuff), plus really significant events and basic self-maintenance stuff (bills, groceries, health care appointments, etc.)
I limit myself to one thing more complicated than picking up prescriptions (right across the street from my apartment) during the week and one weekend day thing, keep one weekend day completely free, and keep at least one weekend completely unscheduled every month. (On those days, I'll do household stuff like laundry, etc. and online stuff but not anything on anyone else's schedule or that involves leaving my apartment building.)
If I'm travelling or have other unusual or strenuous things in my schedule, I keep the weekends before and after completely free.
With those limits, I can be fairly sure I'll be able to keep the commitments I make (barring something like a nasty cold or a migraine or something else that means I can't leave the house at all.)
If I schedule anything above that or near an unusual week, I do it only with people who will understand if I go "Argh, running out of spoons, need to cancel" (which I can usually do a day in advance because I have enough experience to see that coming now.)
3) Invest in things that help me use my energy where it's most helpful.I am not thrilled about the part of my budget that goes toward 'I can't do all the things', but I do it because it gets me more of the parts of my life that I want. (It's hovering around 10% most of the time right now, with money I would not be spending if I didn't have the chronic health stuff.)
I have a cleaning service who comes once a month and does all the cleaning I find hardest to do (bending over makes my lungs extra cranky, so things like mopping the floors and cleaning the bathtub take me a lot longer than they should, even given my usual energy levels.)
This has gotten me back a couple of weekend days a month where otherwise I'd be spreading those things out with 5-10 minutes of cleaning, 45+ minutes of resting, and not having the focus when I was done for writing or other things I really like (and want to do more.)
I get groceries delivered about once a month, usually the week that I have more complicated scheduling or feel lousy.
I budget to order delivery food about once a week, usually on a night that's been a particularly demanding night a work, or when food aversion issues are particularly complicated and nothing in the house looks like food. (Food I did not prepare is more likely to be food, and also I can get delivery for stuff I would not make like sushi.)
I've also worked to figure out the most efficient ways for me personally to handle stuff like bills, making dinner and lunches on a regular basis, etc. Anything I do often, I try to figure out how to optimise so I don't have to keep spending energy figuring out how to deal with it.
4) I track how much I get done on a daily basis This means I have a pretty good sense of how much I can get done on a good day, an average day, and a ugh my body day.
On a really good day, I can do about 5 big tasks that take either an hour or more to do or a lot of focus, plus about 5-10 medium sized tasks that take 10-15 minutes, and 5-10 tiny tasks plus a lot of tiny stuff and some medium stuff I don't bother tracking. If I have more than five big things on my todo list, I need to revise it. On ugh weeks, I aim for 3 big things, and if I get 2 done, I'm pretty happy. Really bad days are basically no big things, and if I get a couple of medium things done, yay.
Grocery shopping in the store is one big thing, a doctor's appointment is one big thing.
More about how I'm doing that here, on my blog, for the curious. - I also find the category approach I'm using there helpful in making sure what I'm doing is reasonably proportioned between different parts of my life and helps me catch stuff like "Wow, you have not done social things with X any time recently."
Overall scaling: None of this is perfect, but it's been a lo more sustainable than thing I've done in the past. I basically plan for me to have about 80-85% of the energy I optimistically think I have, and then have a list of things that are useful but not urgent if I turn out to have more capacity that day than I planned.
(So, yesterday, I had an acupuncture appointment that is effecting the equation of 'what can I do' a little weirdly because my body has commentary about that, but I also have more energy for 'clean the kitchen' and 'cook some things' than I have for a while, so there is cooking going on.)