As I said previously being able to say no is really important when trying to establish boundaries. Boundaries work to protect yourself from other people and is essentially a form of self care. It links to how you want to be treated by others, what you are willing to accept or not accept.
The difficulty with boundary setting is that it is up to the individual to continuously exert them. People will test your boundaries and push against them but it is up to the individual to maintain them.
Having good boundaries is key to having better balanced relationships which ultimately leads on to more fulfilment.