https://positivepsychology.com/great-self-care-setting-healthy-boundaries/#boundaries 

Setting healthy boundaries requires self-awareness. We need to be clear about our expectations of ourselves and others, and what we are and are not comfortable with in specific situations. Setting healthy boundaries requires good communication skills that convey assertiveness and clarity.

Assertiveness involves expressing your feelings openly and respectfully. It does not entail making demands, but it requires people to listen to you. Setting healthy boundaries requires you to assert your needs and priorities as a form of self-care. Tawwab outlines three easy steps to setting healthy boundaries:

Step 1. Be as clear and as straightforward as possible. Do not raise your voice.
Step 2. State your need or request directly in terms of what you'd like, rather than what you don't want or like.
Step 3. Accept any discomfort that arises as a result, whether it's guilt, shame, or remorse.

The third step is common for people with poor boundaries, codependency issues, or are people pleasers.

Sometimes, adults have been raised by childhood carers who've taught them that expressing their needs is bad and selfish. However, not accepting the discomfort that comes from setting healthy boundaries in adulthood means settling for unhealthy relationships that can cause resentment, manipulation, and abuse.