It is such a complicated subject, the 'true self' - now bastardised and manipulated for power plays and relief from anxiety.
The framework I find most helpful is the psychodynamic framework, specifically Carl Jung. Here the 'true self' is all about the unification of consciousness and unconsciousness in a person, and representing the psyche as a whole. So that is unifying all the stuff that lurks in the unconscious with our conscious thought.
The critical aspect of this is that the self is realised through a process of individuation, which means integrating various aspects of your personality. In Jungianism, that includes the representation of the masculine and the feminine - an individuation process intergrates both. So a person has both masculine energies (directive, aggressive etc) and feminine energies (nurture, empathy etc) (The masculine and feminine archetypes are not male and female - indeed men need to integrate feminine energy and women integrate masculine).
If you think about very 'well put together' people, they behave in both masculine and feminine ways - they can be forthright when they need to be, and can be kind and nurturing when they need to be. Their sex at this point isn't really relevant - it is just that they have done the 'work' to get there and inhibit both behavioural sets.
My personal view is that we are interrupting the process of individuation with the trans movement - just because I feel like I am behaving in a masculine way, I am not male, I am more than likely within the process of integration (which is very psychologically healthy!!). In this sense, it really is pure exploitation to take a young person who may be starting to integrate some masculine/feminine behaviours that are not 'typical' of their sex, and tell them they are not in the right body - every body can be masculine and feminine in their psyche. Just not in biology.
Add onto that equation some trauma, some parental attachment issues and even some neurodiversity and it is a dire dire situation