I've been thinking about this thread, as so many of us have, and I figured I might share some nuts-and-bolts stuff about transitioning. Please keep in mind that Cynthia's journey is hers alone, as everyone's is, and the perspectives and possibilities I share here might not apply to her. Well, something probably will...
Physical transition/public presentation
As you may have already read, there will be physical changes. What they don't tell us is exactly how much we'll change. But the fact is, no one knows how much they're change until they go through it. Some folks lose weight, some gain it, some get curvy while some don't. They'll tell you that breast growth is very limited for transfeminine people, more so as they get older, but I'm in my late 50s and have been an exception to that rule, so anything goes. But yeah, most AMAB (assigned male at birth) people experience a level of changes that is easy enough to hide, for a surprisingly long time! And like cisgender puberty, it doesn't happen very quickly, taking 4 or 5 years or more to finish. So there will be opportunities to process changes and figure out when to stop hiding, if that's the goal.
Speaking of hiding, not everyone wants to, needs to or is able to achieve a public-facing transition. I'm coming up on 4 years of HRT and spend about 98% of my time in public as a "male." That works for me. It's easier and safer for now and it is better for my relationship with my wife. I'm fortunate in that I don't suffer as a result. Others wish to paint their nails, wear makeup and female-gendered clothing immediately. Many experience a strong need to do these things, and the healthy thing is to yield to the need. The only person who really knows what's feels right to do is the person who is transitioning. Most of us figure this out for ourselves (with the help of a therapist), then try to work out an understanding with our partner, if we have one. For some it's part time, for others it's occasionally, and for some there's no looking back.
Skin does indeed get softer. Body hair thins, muscles weaken. For most of us, this is the stuff we've dreamed about. Others choose to work out and maintain a harder body. Estrogen changes the calculus, of course.
Hair transplant technology has come a long way. For me, estradiol grew back a lot of hair, but transplant surgery did the heavy lifting. But there are excellent wigs out there and many wear wigs full-time when out and about. But I've encountered others who don't feel the need to deal with their hairline, which is great. Certainly, that doesn't determine someone's gender.
We do all of this stuff because it feels right. It feels good. Euphoric even. The thrill of it all can cause a confusing reaction for many, as body parts respond to the rush of excitement. Some feel very dysphoric about original equipment "parts" and seek gender confirmation surgery, while others are OK with all of it.
Facial feminization surgery is a bit of a process and it can be expensive, but many states require insurance to cover procedures under the "gender affirming care" umbrella. This can be a big deal for some, as the years of testosterone creates contours that are difficult to cover with makeup.
The reason for thinking about/doing a lot of this stuff is twofold:
1. Feeling good about yourself and wanting to see "her" in the mirror
2. Wanting to "pass" as female, to be seen as a woman by others
#2 is often the case because it's easier and safer in some locales to just blend in. And it sure feels good. Some folx are out and proud and refuse to comply with cisnormative standards for appearance, dress and behavior and don't worry too much about passing, but that's not for everyone.
Going out in public the first few times can be terrifying. Don't be surprised if she gets all dolled up, then backs out right as she's putting her foot out the door. In my opinion/experience, pushing her and telling her not to worry doesn't help. The best thing you can do is to let her lead.
Emotions/moods
As I understand it, estradiol doesn't make people emotional or bring on mood swings, but swings in hormone levels do, as bodies that were previously wired to run on testosterone are no exception when Mother E knocks on our door. Along with some of my friends, I've had to check myself when feeling extra weepy or fatalistic and remember "oh yeah, today is the day I'm supposed to change my estradiol patches." Conversely, I've had days in which I felt cheerful and calm and realized "this is that time in my medication cycle in which I think my levels are just right." This might be something you'll both need to be mindful of. As a cis woman, you're probably much more prepared to deal with it than Cynthia is at this point.
Intimacy
I won't say much about this, except to point out that Cynthia may experience waves of new feelings about herself that manifest in fears, desires or even apathy. Love and communication will surely help. Please understand that time is your ally. Giving her time to adjust and possibly figure out what she wants and needs, and how she's wired might be required. There will be changes, but living as one's true self usually connects one to their bodies in new and exciting ways.
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The most important thing to remember is that, as others have said, this does happen pretty slowly. Neither of you need to figure this all out right away. Give yourselves (and each other) the time and space needed to process what's happening. If you're fortunate, you'll be able to be present at a time when new and exciting changes are occurring. There will be rough days, but there are always rough days in life. The good days make it all worthwhile.
I had this on my mind and just spewed it out here, so it may or may not be stuff you're interested in, so please disregard if I'm being pedantic or irrelevant.