How do I set boundaries without feeling guilty?

Be specific and behavioural rather than accusatory, state what you will do rather than demanding others change, expect discomfort initially, and hold the boundary consistently. Guilt usually signals unfamiliarity, not wrongdoing.

Stating what you will do rather than what the other person must change tends to reduce conflict — 'I can help on Sunday' rather than 'stop asking me on weekdays'.

Expect initial pushback and discomfort. Guilt usually reflects unfamiliarity with saying no rather than evidence that you were wrong to.