There's a couple people out there who need this one.
Setting boundaries can be hard, but like any other skill it gets easier with practice and time. No one gets on a treadmill for the first time and sets a world record. It takes practice and working at something to get better at it. Same thing here. Practice setting those healthy boundaries. Practice saying no when it's in the interest of your health and wellness. The payoff is less stress, a mind that is more calm, and healthier relationships.
Not sure how to start? Consider speaking with a trained therapist who can help guide you.
Keep practicing. You can do it.