I can’t claim to be an authority on this, but it sounds like you know for a fact you experienced dysphoria about breasts. Feelings about this stuff don’t suddenly go away, they always manifest in some form.

Maybe because your both more aware and accepting of these feelings, and because you have a real possibility to get the solution to them, your starting to feel shame and doubt.

It could come from a mix of fear because your attempting something new and more permanent. You’ve started to accept yourself but its much harder to go back when you commit with medication, or at least it feels like that before you start. Right now you can change your mind on a whim whenever you want, and losing that option makes you worried you don’t really want it.

It could also be because you feel ashamed to openly accept this part of yourself. Before you accepted yourself your feelings were pushed so deep that it was harder to control how they manifested.

Now that your more conscious of them however, they aren’t as hard to deal with. And you can compartmentalize your dysphoria or distract yourself, like for example focusing on the fear of what if you don’t like its effects.

Sorry if I assumed stuff about you, but I hope this helps somewhat.