Hey! As a fellow trans person diagnosed with depression and other mental illness, here's my take on it.
Transitioning for me definitely eased some of my depression symptoms, but If you have a mental illness transition isn't a way of curing that. Often that's a chemical brain thing that can be influenced by transition but isn't caused or cured by it.
For me, transitioning was about becoming who I was. It made me happier than ever before, and more comfortable with my life. Being called he instead of she, being a father one day, being a boyfriend, etc those were all roles I craved.
Do you feel like a woman or a man? Or neither, or both, those are valid as well.
Transitioning isn't just because you're depressed. You can still need to transition if you're not depressed. Everyone's path is different. I hope I've helped a little bit, good luck on your journey!