Psychiatrists special in the diagnosis and treatment of mental disorders. Crossdressing is no longer considered as a mental disorder. You can’t be cured by using medication, because it is not a treatable disorder. Please explain this to your girlfriend and encourage her to do some research on gender issues to put her mind at rest.
If counselling is still being considered then you need a psychologist (NOT a psychiatrist) who specialises in behavioural and personal issues.