For me I use visceral sensations to tell me when to stop. Like a burning feeling in my head or my body feeling like it just ran a marathon. I've also experienced where I tried to push beyond what my willpower will allow. Then my brain will just start forgetting things - I've lost my phone, my bus card, my wallet in these scenarios, and everything starts taking twice as long than they should and I'll keep fucking up which requires even more time to correct my mistakes. I have no control over any of these things. Lately i just try not to go beyond my limits because my brain just rebels and I have no say in any of it.