Saying no is important when respecting yourself and others. I used to say yes to everything which led to resentment and anger held to the other person because I blamed them for asking me to do things I don’t want to do and I felt obligated to go and do them.
You only owe yourself anything. You don’t owe anyone for anything. You have to put yourself first and realize that sacrificing your health, money, mental health is not worth it.
Just remember this when your feeling blue when you say no it’s better there angry for a night if it means peace within you.
It’s one of the most valuable things my counselling taught me.