OP,it’s very easy to be put on the spot especially if you’re not used to it and say „yes” even if it’s detrimental to you.
Look up some assertiveness courses (I bet you find something on YT) and practise saying „no”
The phrase that is very useful before you commit yourself to do anything for anybody is „let me check (diary,calendar,whatever)and get back to you”.
This way you’re not saying „yes” straight away and it gets the request off your back for a while.It also gives you a chance to think it over and reply in writing- email/message thus avoiding face to face confrontation and uncomfortable feeling.
You might encounter some people that are very persistent in pursuing your time/energy as they are not used to you saying „no” to them. Rinse and repeat „let me check…and get back to you” with „I said let me check and get back to you”.
Last repeat might seem to be a bit harsh and rude but this is the requester’s rudeness redirected at them.
As for family and friends you might be met with sulking,arguing and verbal agression. Turn around and walk away (or hang up if you’re on the phone) after saying „don’t speak to me like that” then DO NOT contact them first as it will be seen as your capitulation.
Never,ever say “sorry” with your refusal as it indicates you feeling guilty so they will try again.Don’t offer alternatives either if the request is detrimental to you.
The world is not going to end,the sky won’t fall on your head if you say “no”
As with everything else it takes time and practice. It gets easier with time 🙂
Then it’s straight to “No,it doesn't work for me” “No,I can’t do it” and “No,I don’t want to do it”
Good luck. Stand up for yourself as nobody else will 💐