There is an unspoken assumption underlying your thought here that may, if brought to the surface and challenged, help clarify what you think - and hence inform what you do.
The assumption, simply put, is that there is such a thing as being the opposite gender. We can make sense of being the opposite sex (whilst knowing it to be impossible), but it is not so easy to make sense of talk of being the opposite gender.
(An aside: in fairly recent times, 'gender' was often used synonymously with 'sex', a usage usually motivated by a kind of squeamishness about sex as an activity. Unscrupulous activists sometimes use this synonymity to confuse, deliberately or not. We should avoid that here; I distinguish 'sex' and 'gender' above and in what follows.)
OK. Now think about beliefs. A young person may mistakenly believe she is overweight, and try to avoid eating, injuriously to her health. A solution (not necessarily easily achieved) may be to convince her her motivating belief is false.
Likewise, a young person may believe she is the opposite gender, and perhaps seek to bind her breasts or whatever, injuriously to her health. Again, a solution (again, not necessarily easy) may be to convince her her motivating belief is false.
The cases seem similar. Both involve false beliefs. But I want to suggest there is an important difference.
In the first case, the belief is indeed false, but it could have been true; it makes sense (though it be false) for that person to believe she is overweight. The belief in question, we might say, is thus only contingently false.
In the second case, and by contrast, the motivating belief is necessarily false; it could not have been true, since there is no such thing as being the opposite gender. (Here is a point at which "'gender' is not a synonym for 'sex'" is important; of course there is such a thing as being the opposite sex.)
Contingently false belief vs necessarily false belief as causes of pathology: should these give rise to different ameliorative strategies - different therapies? It would seem so.
The second case - belief in opposite gender - is at issue here. Perhaps, given the modal status of the motivating belief, a therapeutic strategy emphasising the semantics of the belief is called for. In simple terms, this might consist in starting with questions (to the young person) about what she means by 'gender', 'opposite gender', 'sex', 'identity' and so on, and going on to investigate the sensicality of what she claims to believe about herself.
Of course this is likely to be difficult. Any young person claiming to be the opposite gender is very tangled in her thought; such thoughts are notoriously difficult to disentangle. But it does seem to be called for - and, yes, be assured it can work.
Such a strategy, note, in concentrating on the sense of the motivating belief, differs substantially from what might be best in other cases where the motivating belief for the pathology in question is contingently rather than necessarily false. In such cases, the truth of the belief is required to be challenged from the outset.
So - tldr - although both cases involve an aetiology of false belief, the differing modal status of the beliefs in question may imply different therapeutic strategies. (And 'opposite gender' has no sense, albeit that 'opposite sex' does.)
I hope this makes sense (!) to you, OP and others. And I hope it may help in some small way with the difficult situation(s) you find yourself/yourselves in.