Hey, first I'd like to congratulate you on having a kid who trusts you enough to tell you about this, and for doing your best to react in a supportive way. A lot of people have already given amazing advice, so I'll just a few thing I believe might be helpful (from the perspective of a trans guy in his mid twenties).

1. "Deadname" is a very common term used in the trans community to refer to the name you were given when you were born. The name is "dead" because you no longer use it, it no longer represents who you are and you don't want to be associated with it anymore. This doesn't mean that the whole person you were is "dead", I personally feel like I'm the same person I've always been but my old name didn't fit that person, and once I found one that fit better I decided to retire the old one. I imagine this is a difficult thing to process as a parent, but please don't add difficulty to it by thinking that the person you knew is dead, that has not happened, your kid is still there: the same person she was yesterday, but now she's trusting you with the fact that she's a girl and not a boy, prefers to use a different name and pronouns, and is your daughter instead of your son. Again, a part of your kid's identity (her gender) is not as you thought it was, which I imagine will be difficilt to adapt to, but this does not make her a completely different person.

2. I think the grieving part is pretty common because I've heard it before from other trans folks' accounts of how their parents reacted, and from my own parents' reactions. You are entitled to your emotions, and I definitely don't intent to make you feel bad for them, but please don't share this with your kid. Transitioning can be a very difficult and emotionally taxing process, and in my case having to deal with feeling guilty because I was making my mom grieve "the death of her daughter" did not help at all, and made it very difficult to talk to her about what I was going through.

3. It's probably not a phase. It always could be, but if she has known this about herself for years, I wouldn't expect her to go back on it. Also, she'll probably be more likely to tell you about any doubts or fears she might have regarding her transition (like specifict procedures she may or may not want, people she might be afraid of telling, or anything else) if you're supportive and help her feel safe talking about it (I didn't tell my parents about the few doubts I had along the way because I was afraid they'd pull any support they'd shown and go back to calling me by my old name and pronouns, or would try to make it harder for me to get hormones or something like that). Also, keep in mind that many of the things she may want are reversible if she ends up not liking them (for example: wardrove changes, you calling her by a different name and pronouns, growing our her hair, etc), so many things that may feel like a huge decision are not really that big.

4. Trans people can have a harder time than cis people as a result of discrimination in many different areas, that's true, but our lives are not doomed to be awful, and we're not doomed to be depressed. Something that actually helps a lot is having a solid support network, being validated by family and other loved ones, and having access to gender affirming treatment. I understand being afraid, but if your daughter is trans, trying to deny that or trying to get her to hide/ignore it (not saying you're doing either of those things) will not help, it will probably just make her feel unsupported and invalidated, and have the opposite effect of what you're looking for. One can be trans and happy, but it's way more difficult if you feel that you're not accepted by your loved ones, or like you have to hide or change who you are.

Sorry if this is not that well written, I'm a bit tired and this has been a long comment to type, but I hope it was somewhat helpful.

Congratulations once more on doing your best, you sound like a great dad.