This is the place to ask, since most people frequenting this section have been through some version of this.

First of all, one of the most terrifying times is right after you get that news - there's a million worries all vying for space in your mind, there's no real idea of what's next, and people are lining up to tell you about the doom and gloom.
I was afraid of many of the same things when my wife dropped this bomb on me six years ago. In the end, she is still basically the same person, except that we all grow and change over time (she's not the same person she was 10 years ago, either). Odds are that you will still recognize most of the important personality traits in your partner afterward. However, that doesn't mean there won't be a period of experimentation - like a teenager trying out new identities - that can be unnerving as it's going on. There's no guarantees about your attraction, unfortunately. Plenty of people find that their orientation is flexible enough to make an exception for one person they were already in love with, and there are quite a few lesbians with trans men and straight women with trans women out there. However, not everyone can make that jump, and you shouldn't feel guilty if it turns out you can't; you also cannot control your attractions. That's a long way off, though, and you have some time to see how things play out first.
I also had to go through a period of grieving for the husband I lost, even as I welcomed the wife she was becoming. My advice is always to give yourself space to feel what you need to, communicate as honestly as you can with your partner, and remember that as long as the answer to "Do I want to be in this relationship?" is "yes" there's still hope. (If the answer changes, you'll have to reconsider whether you want to keep trying. But there's no point in borrowing trouble about what your future self might feel, as long as your current self is still committed to the relationship.)
(About half of all relationships do survive transition, according to the only major study I'm aware of on the subject.)