A boundary is not a wall. A wall keeps everyone out. A boundary tells people how to be close to you safely.
Why boundaries feel like betrayal at first
If you grew up needing to stay agreeable to stay safe, a boundary can feel like you are doing something wrong, even when you are doing something healthy.
You are allowed to disappoint someone and still be a good person.
Practice on something small first. The goal is not to get it perfect. The goal is to prove to your nervous system that saying no does not end the relationship.