A central aspect to peace of mind is accepting one's self - what and who one truly is, physically, mentally, emotionally, and, if applicable, spiritually. This acceptance can bring with it a release from constantly worrying about whether one is "good" or acceptable enough to others and their standard. Self-acceptance also empowers one to choose their own their own standards, boundaries, and expectations for who they are, wish to become, what they seek to achieve, and healthy behavior (versus working to reach unattainable or unhealthy goals). Finally, this acceptance opens a doorways to truly loving oneself.
However, In order to accept one's self, it is necessary to know one's self - which can be quite a task, and also a paradox - if one doubts themself, how can they accept what they might learn? Logic, feedback from others, intuition, studying, emotions, and other sources can all provide various (and conflicting) information.
Ultimately, though, to accept one's self, one must take a risk in choosing to trust and believe their own judgement, knowing that it might not have ideal outcomes (though not necessarily the most undesired ones, either) - they must genuinely self-express, take what comes with the experience,and see how, if at all, it fits into one's life, and who they want to be. It may not be easy, and psychological conditions can affect certain aspects of striving to achieve this. However being one's self is essential to being in a close, healthy relationship - both with one's self and others.
This being said, what one learns about one's self may not always be what they want - sometimes these aspects can be changed, or improved, and sometimes they cannot - what is important, however, is that they be accepted, as they are a part of one's self; otherwise, by rejecting them, one is rejecting an aspect of who they are.
Self-expression is very vital to self-discovery, be it in one's actions (including volunteering), body art, words, actions, wardrobe, self-presentation, or other ways. In addition, such can help give one goals and a purpose to focus on, with significant results (beyond just be who one is and loving themself); self-expression involves showing and presenting one's true self - not necessarily to the whole world, or even necessarily one's closest individuals - sometimes, just being open and true with one's self about something can be enough.