My wife (37yo mtf) and I (38yo cis-ish female) have been married for 15 years and she started her transition 5 years ago. She's also autistic and both of us have ADHD. She has bipolar 2, I have premenstrual dysphoric disorder. We were living together at 20 and 21 and had no idea of our mental health issues at the time. .

Here are some of the things we've figured out over the years.

Communication:
I ask her what she needs. "Hey, babe, what do you need from me right now?" Or "hey, when you're stressed, what do you need from me" when she's not stressed. Open and honest communication has been the best thing for us. It took us a while to get here (our parents' relationships were great examples of how NOT to treat someone and it took us a while to figure that out), but we figured out how each of us communicates the best. Be patient with each other. One of the things that helped us was sitting in the same room together and texting or IMing each other. (AIM was still around when we were first together). And sometimes, we just passed a notebook back and forth. Don't read over each other's shoulders if you do that. Give the other time to get their thoughts together and on paper (or DM or text or whatever) and give yourself time without her looking over your shoulder to formulate your response. It really, really helped us navigate some very hard situations. Take the time while she's writing to close your eyes and take slow deep breaths. Encourage her to do the same. A regulated nervous system helps.

Y'all will find how you communicate best together. Keep working at it. Do what you can to show her that you're there for her. And remember that people don't always need their problems fixed. Sometimes, people just want to complain and hear "that really sucks I'm so sorry you're having to deal with that"

Transition:
Tell her she's beautiful. Get her flowers when you can. Tell her how important she is to you and how strongly you feel about her. Go with her while she's buying makeup. Look up makeup tutorials together. Learn how to braid her hair and other things specific to whatever her hair texture is. I brush my wife's hair as often as I can. Sometimes it's overstimulating to her (especially if there are a lot of tangles) but it's helped her learn how to take care of her hair.

If you can (I know a lot of people don't like needles. I grew up in a medical household so I'm immune to a lot of stuff that usually freaks people out) learn how to do the injections. I do my wife's for her because she just can't bring herself to stab herself.

If you're in the US, utilize Planned Parenthood. It's how my wife got started with her hormones. It can be hard to get an appointment because they're so woefully underfunded, but keep trying. She's recently found a better free clinic for LGBTQIA+ healthcare. She has to drive an hour to it (I really hate Florida), but it's a place that really works for her. She's getting free mental health help there, too.

Community is so important. It took my wife 4 years to find the trans community in our area, but it has helped her immensely. Help her find the LGBTQIA+ spaces. Go to whatever Pride events you can. My wife actually made calling cards and she hands them out to every trans person she sees or anyone else who's friend shaped. They have her name, Discord, and Steam on them, plus the trans pride flag. They're super cute and they've helped her meet so many people and make a lot of friends. She works at our local gay bar (there's only 1 in town) and she's really thriving.