Feeling guilty about setting boundaries is incredibly common, especially for people who were raised to believe that saying no is selfish or that they should always put others' needs first. But this guilt is based on a misunderstanding of what boundaries actually are and do. Boundaries aren't walls to keep people out or ways to punish others - they're guidelines that help relationships function better by creating clarity about what you can and cannot do.
When you don't have boundaries, you often end up overextending yourself, which leads to resentment, burnout, and ultimately less ability to help anyone. Healthy boundaries actually improve relationships because they prevent the buildup of anger and exhaustion that comes from constantly sacrificing your own needs. The guilt you feel is often your inner people-pleaser protesting, but remember that you can't pour from an empty cup. Good people will respect your boundaries even if they're initially disappointed. Those who react poorly to reasonable boundaries are showing you that they valued what you did for them more than your well-being.
Start with small boundaries to build your confidence, and remind yourself that taking care of your own needs enables you to be more present and helpful to others in the long run.