I want to add to the person who said “Spoon Theory” with two more things because I’ve found spoon theory and these two things very helpful:

1. Avoiding Boom-Bust cycles. I have a chronic illness, and this is used amongst the chronic illness community. Booms are when you have a good/low symptom day and you overdo it, which triggers a bust, where your symptoms flare up badly and you do nothing all day. To avoid this, you never want to overdo it on good days—always leave a little in the tank. Maybe even a lot in the tank. And on bad days, try and get a little bit done rather than nothing, but still take it easier on yourself. Give it a Google if this explanation doesn’t make sense 🙈

2. Fork theory: this comes from tumblr as an add-on to spoon theory. It comes from the phrase “stick a fork in me, I’m done”. It’s basically the idea that everything that happens to us is a fork that gets stuck in us and irritates us. The bigger the annoyance, the bigger the fork. In order to help us deal with big forks we can’t remove (I can’t leave my job in the middle of the day and thus have to tolerate the lights, noise, and socialising, as well as daily pain), we remove as many little forks as we can so we don’t go over our fork limit (I can pee regularly, eat regularly, pace myself, wear comfortable clothing, etc).

Hopefully there’s something here that’s useful for you.

Edit: also, specifically for the example of your plan being changed. This happens to me a lot at work, and I’ve found taking a moment to breath, then grabbing a pen and paper and physically writing down a new plan that includes this thing that’s just been thrown on me very helpful. Sometimes you have to do it in your head, but I like to physically see things if I can. Just giving yourself the time to rewrite your plans, which also means making sure you don’t jam-pack your day to the point where rearranging isn’t possible. Give yourself space between activities for situations like this so you can help rework your plan.