I take a break if I've lost more than 50 points that day. Any less, and I try to regain what I lost. It doesn't always work of course but if I have that hard rule of 50 points, it reminds me that some days are just not my day and I should go do something else for awhile, maybe come back to it later in the day if I want to. But yes, I've been exactly where you are now before I placed that limit, or when I disobeyed it!
Hazards of thinking when tired...

50 points sounds like a good threshold; mine has been more like 100, which is clearly too high. It's curious how one's performance can vary day by day, even if it's the same brain doing the work. Fatigue, lack of sleep, and mental preoccupation can all skew one's performance downward.

Hi! My name is Lauren Goodkind and I’m a respected chess coach and chess YouTuber based in California:
https://www.youtube.com/channel/UCP5SPSG_sWSYPjqJYMNwL_Q
To limit a loosing streak, I make sure that I'm in the right mindset and I'm fully alert. If I'm tired, then I tend to make silly mistakes.

It's reassuring to know that comparative beginners like me aren't the only ones who suffer from this problem! (Even grandmasters have off-days, rumour has it.)

Here's a happy postscript to the description of the original problem; not allowing my rating to fall too far in a single day seems to help. The 50 pt threshold definitely works better for me than 100 or so.

Nice! I use the 50 point threshold for games, 200 for puzzles 😅 thing is I can gain 100 and lose it again quite easily...

I'm happy to report that my puzzle progress has continued reasonably well, thanks to everyone's good ideas here. I've discovered a new sort of hazard while doing chess puzzles: having a mild illness that gives one a bit of mental fog doesn't help with puzzles any more than any other kind of fatigue does! No surprise there, of course. I've got a very mild case of the coronavirus at the moment (thanks, Omicron BA2!), fortunately nothing serious at all, just a slight nuisance. The current dip that I'm experiencing is probably just normal fluctuation, of course, but I'm happy to blame the coronavirus if I can credibly shift any blame!

I'm beginning to think that there is indeed some evidence of covid brain-fog at work here, even though I have only a mild case of the bug. Doubtless the trend will reverse itself at some point once I start to recover. In the meantime, I'm going to take a few days off from puzzle-solving before my rating falls into negative territory!

I'm happy to report that my puzzle progress has continued reasonably well, thanks to everyone's good ideas here. I've discovered a new sort of hazard while doing chess puzzles: having a mild illness that gives one a bit of mental fog doesn't help with puzzles any more than any other kind of fatigue does! No surprise there, of course. I've got a very mild case of the coronavirus at the moment (thanks, Omicron BA2!), fortunately nothing serious at all, just a slight nuisance. The current dip that I'm experiencing is probably just normal fluctuation, of course, but I'm happy to blame the coronavirus if I can credibly shift any blame!
No discussions of coronavirus in the chess forums.
Sometimes it's better to avoid doing puzzles when you're worn out! I have a knack for persisting stubbornly when it would be better to stop and take a break. I've managed to fall 300 points in Puzzles over the last few days, just by missing things that I might have seen by thinking more slowly and calculating more accurately. The streak will reverse itself, no doubt, but I do miss being above 1600, however briefly I was there -- now currently in a crater at 1348. The worse I'm doing, the more pressure I put on myself to reverse the trend, and then I simply keep doing worse and getting frustrated, all the while disobeying my usual rule to stop for the day after declining a certain amount. It's very silly, of course!
What's your way of limiting a losing streak, either in puzzles or games? (Apart from not losing, of course.) When do you decide to take a break?