I know exactly how you are feeling - it is awful knowing that you only have a couple of years left and then they will be off.
BUT - you can't tell her that. Sje cannot be made to feel guilty that her mother is having mental turmoil at the thought of her leaving. I thinkl the best approach is encouragement for her to spread her wings, and to leave without a backward glance, but all the while knowing that she can come home any time.
Oh I don't know really, I am just making it all up as I go along like the rest of us.
But don't think that it will necessarily ruin your relationship - apologise fully, then just park it mentally and move on. She will be miving away from you - dd goes through episodes where she will NOT talk to me about anything, just wants to talk to her mates - and I think that is normal. It is not a rejection of you and your ways.
I know you probablty think it was rude for me to say to get therapy and of course what the fuck do I know, but I still think it might be a good idea to talk to someone about how your esteem seems to be bound up with what validation you get from others.
The only advice I can offer with stroppy and arsey rudeness is just to be zero tolerance about it - say if you say xxx I will take your phone away for the day/switch the internet off/you are grounded one friday. If you sit her down when all is calm and just set the boundaries like that you can follow them through, and she can't argue. When dd acts up I warn her, and if she carries on I just say calmly 'I will have your phone please'. She will slam it on the table and retreat huffing and puffing but it does work, she has a think (she has to, she wants the bloody phone back!) and calm is restored. Just short sharp shocks really, like you do with toddlers.
I hope all goes well with you both, I really do.