Yes, this, @EvenMoreFuriousVexation you said it so well, my ex does these kinds of things to our children. He tells them that he has no friends and that they are the only people he sees so they better come to visit every week. My youngest (17yo) came to tell me this and even she commented how this is not fair and is totally inappropriate of him to lay this kind of burden on them. She said, "he should be talking to his friends or a therapist not us".
Anyway, OP you did the hardest part, you had "the talk" with him, but this next while will be hard too. I lived with my ex for four months after I said I wanted to separate. I slept on the floor in the same room this whole time terrified that one of the kids will come in one morning and find me there. It was very difficult and emotionally draining and because we did not tell the kids right away so I had to hide it from them. However, it will all pass, and you will get to the other side.
Luckily for you it is "only" three weeks. I did everything I could think of to help me get through it. I was going to therapy, I meditated, did yoga, prayed (not religious at all), I posted on MN… a lot, went out as much as I could, met with family and friends and took my kids out for treats. (…and drank a lot of wine I am afraid 🙄).
When it comes to your child, I strongly believe that the kids (no matter the age) will mimic your behaviour and will follow your feelings. You daughter is upset because he spoke with her and upset her. You need to talk to her and reassure her that everything will be ok, but whenever you talk to her stay calm and composed, do not show emotions, do not catastrophize the situation. Divorce is not something people want to happen of course, but it is not the end of the world either and you need to communicate that to her. Just tell her in simple terms that you and your husband do not get along so well anymore and that you are not happy. Stay strong and keep posting.💕