As much as it is easy to blame others for you saying "yes", and many times those others can even be our own kids...it is YOU my friend who holds that responsibility to honor your threshold, capacities and limitations.
You know them best.
Your family. friends, co-workers, etc. don't know you like you do. They're not mind readers or body readers where they can tell how high your physical or mental pain is and when you have reached your limits...but you do.
And when you reach them, ask God if whatever is in front of you is something He wants you to take on and pray for strength to do it. And if it's not, ask for strength to let it go and rest.
You are your best advocate. And when you struggle to say "no" for identity reasons...know that you have THE ULTIMATE BEST advocate on your side reminding you who you are despite any yes or no you ever give to anyone.