Crossdressing is not a diagnosable mental illness, Taylor. Gender identity disorder was written out of the DSM's latest revision.
While those with gender identity issues have significantly higher rates of mood disorders (depression, anxiety, PTSD, etc.), I am inclined to think disrespect and abuse by others and not accepting oneself as they are is a significant part of the problem.
I've been told by a psychiatrist that a cause and effect relationship has been found between disrespect/abuse and mood disorders.
The best thing you can do is accept yourself as you are. I realise this is easier said than done. It does require significant thought and work.