I've been through it, and I was terrified of the exact same things. (Except it was my wife starting estrogen, but I still worried that she'd change totally, or stop being attracted to me and leave me, and so on.) They are not petty thoughts! Those are huge, scary, reasonable worries.
There are no guarantees; that's the ugly truth out of the way first. With that said, my wife turned out to be just the same person she was before, only happier, calmer, more centered, and *more* loving because now she wasn't fighting herself all the time.
And most of the individual people I have known during transition have been much the same. There's this pernicious narrative that says that a person who transitions automatically must or will transform into someone barely recognizable to their loved ones. I'm sure it does happen... but not nearly as often as claimed will be the case (which is always!). Personally, it seems to me as if sometimes transition is a good *reason* for someone who already wanted a change to choose to remake themselves completely - the same thing happens sometimes when someone changes careers or moves to a different country - which is perfectly valid, but doesn't imply anything at all about whether someone who is otherwise relatively content with their life and themselves WILL change so totally. Hormones are not a magic personality transplant drugs; if it were true, every cis person who had their inner hormonal balance majorly altered would become someone new, too. It *can* work that way (yep, even for cis people), but it doesn't have to.
With all that said, you know your boyfriend best. All you can do right now is hang on for the ride, and know that some of us out there know exactly what a roller coaster it can be. Good luck, and don't forget to take care of yourself too.
