Pretty much, the "true self" was a big representation of your inner demons. Everything the shadows talked about being symbolised with a boss fight. A persons shadow comes out in the mirror world, claims to be you, and starts rambling about something that is technically true about your character, but kind of exaggerated. In the games, one character's shadow pretty much inferred they were gay and desperately wanted someone to accept them. The character vehemently rejects the idea, and thus the shadow, and it gained power from that. That character's true self looked like a big muscly dude with a ring of flowers around it's neck. In other words, a big gay symbol XD Your character pretty much just has to have a problem of some kind that the shadow exaggerates, and then when rejected, transforms into a monstrous representation of that issue.
Think first of something in your character's life that they aren't all that cool with, then how that could be turned into something extremely negative for a shadow to spring from. It doesn't have to be perfected right now, and it will become more clear as other people go through the motions.