So tell me how you feel when you have orgasm when dressed as girl? Very exciting? Highly depressed?
So what to do? Firstly you will not touch your sissy clit when you are dressed as girl, that is something your Mistress/ Master can do for you. If you are turned on you can hump a pillow or anything you like. But no touching your sissy clit with your hands from now on.
Second thing you need to learn is right things way to orgasm. You should only cum when you are sucking your favourite dildo or have something in your mouth to suck on, without touching your sissy clit.
Third, you will imagine about cocks, off course not your cock silly, that is not acceptable. Think about cocks of your friends, role models and sexy hunks.
If you cannoṭ ejaculate first like that keep practicing. Look at some girls sucking cocks to keep you motivated.
You are noṭ allowed to ejaculate until you are not doing one of this three things.