The depression part is because when you transition - you're promised access to a warm, welcoming, and inclusive community. It's literally sold as something that will cure all of your brain problems, life problems, relationship problems, etc.
This is likely one of the last and most fatal interaction a transgendered person gets to have regarding their community. A shocking realization that the actual community does not care about you at all and that everything you've done is basically irreversible. Literally you can't garner up enough care for yourself for a suicide hotline to give a shit about you.