Understand that having boundaries and saying no does not make you a bad person. Not at all. When you enforce your boundaries you are protecting yourself and that is to have self-respect. It takes self-discovery, really knowing yourself, to identify how and why this might happen to you and how you can improve. You can explore this in therapy, coaching, groups, retreats etc. You can also take this journey on your own, but having someone you trust to support you is always the better option.