That's not stupid at all, and it's perfectly normal. In fact, it's even TRUE that you are losing your girlfriend - you haven't lost the beloved person (thank goodness), but you are in the process of losing the "partner as recognized by society," if that makes any sense. You're going to be treated differently, as you've just discovered; you're going to have to cope with people who assume that femme = not lesbian and not be able to correct them without a long awkward conversation that they probably won't believe/remember anyway.

I sympathize; I went through the process from the opposite direction, but it's been four years and I still caught myself this weekend wishing I could say "my husband" to some random repair guy who's in my house with me alone. Because regardless of how much I love the person who is my wife, The Husband as a concept has a whole set of societal expectations and baggage that sometimes worked in my favor and that I miss. And I've been lucky enough that I largely prefer women, so while the assumptions about my orientation are *still* wrong (I'm bi), they're less irritating now.
I imagine it must be much worse for you, who's not generally interested in men and has spent years correcting people's belief about the Invisible Boyfriend, and now you have to actually confirm it. You weren't ready, and you were forced into it by a conversation you've had and disliked dozens of times over the years. It's no wonder that you came away from that frustrated and sad, because at a time when you desperately wanted to be able to correct them, you had to give up and admit you have a boyfriend.
Lastly, "this thing hurts" does NOT mean you're not ready. It means that you're human, you can't control your emotions, and sometimes even expected stuff hurts. You're handling this amazingly well; you haven't flipped out on her* or decided you can't take this. It was just upsetting, which is real and valid but doesn't mean you won't get through it. (Yes, it will get more comfortable with time, but that's not really the point here; it's not about the "boyfriend" aspect, it's that you do need to respect your own feelings and take time to grieve the things you will be giving up in terms of interacting with the rest of the world. It's every bit as hard to be a lesbian who is continually mistaken for straight as the reverse, and a thousand times harder when that's happening before you're fully prepared.)
*respecting your agreement with your partner on pronouns, given that it's sort of the point of this post