all have to do is stop saying I'm not a girl.
If you want to feel out-of-place, yes. If you have a male body and you say you're a girl, you're gonna have a bad time. Even if not for social reasons, your body is there. Denying it is will be stressful.
George, transgenderism is a complex thing and is generally not something you can just go "Oh I totally don't feel like a girl". Saying to not worry about your body is like telling someone with anxiety to just "deal with it" or telling someone with depression to just "be happier". And this isn't just me talking, this isn't just a fad talking, but almost every psychological and psychiatric organization, including the American Psychological Association, claim that transgenderism is something that, when given distress like hers, is a mental disorder and is treated like any other, something you can't just go "OH hey, I don't want to be transgender, so therefore I'm not!".
The thing is, George, I wouldn't normally talk to you about this, but transgenderism can be a huge problem, people commit suicide for not being the body that they associate with. What this person needs is professional psychiatric help on the matter, not "Oh just deal with it".
Whether or not it is a social thing or a biological thing doesn't matter, social anxiety is purely a social thing and it can create a huge amount of distress for the person, and again is something that can't just be fixed by saying "Oh fix it". The point is Cecil is feeling a lot of distress, and just saying to suck it up, I don't think, and neither does most of the Psychological associations of the world (which know much more about mental disorders than you or I) is gonna help.
EDIT: And just to add to that fact, the DSM-V, the Diagnostics Statistics Manual, largest and most cited book for mental disorders around the world,
claims that based on monumental evidence the best way to ensure a good standing of living for someone experience gender dysphoria is gender reassignment or social and legal recognition of their appropriate gender, or therapy. None of these options say to just have them "deal with it".