(My wife transitioned about 4 years ago now.) The thing that leaps out at me is "becoming a straight woman" - there's two aspects to that, and it's entirely possible that you can cope with both, with one and not the other, or with neither. You are going to be *perceived* as a straight woman by much of society (and I'm bisexual, so I can attest from personal experience that almost nobody ever sees bi as an option...). The second half is that you do not, however, have to accept that label for yourself. There are even self-defined lesbians who have occasional male partners or relationships with men. If your partner is OK with it, you can continue to see yourself as a lesbian with one exception, or you can accept a label like bisexual/pansexual/queer rather than lesbian OR straight.
On the other hand, if you genuinely cannot imagine being with a man romantically or sexually, then you're in the same heartbreaking boat as het women whose husbands transition - you just can't change your orientation enough to include him. In which case, yes, you have to either choose to remain in a relationship with someone you can't be romantic with, or move on.

I will caution that since you love him very much and don't want to leave, it may be worth running a trial period in which you see if you can cope with the changes. Perhaps you'll find your orientation is more flexible than you expected at the end of it. That's only if you think you can tolerate a long time in limbo waiting for the other shoe to drop, though, and no one should blame or accuse you if you can't. This is a hard thing to deal with even for someone who *is* sure about their attraction to people of the same gender that their partner's transitioning to.
You are not being selfish, and compromise *may* be possible if he's willing to work with you here.
Lastly, I will say that my wife is indeed the same person. She's happier, calmer, and more contented now, but there haven't been huge personality changes. The physical changes ARE huge - and I'm not in any way minimizing how much that can matter - but I was terrified that I'd also find myself married to a stranger, and that didn't happen.