It's incredibly hard not to let this happen. You're upset and stressed and you can't spend every minute of time with the kids pretending you're not.
I suppose you need to explain that you're stressed and upset but don't give her the details so she feels part of the situation. Just tell her you're having a hard time but you want her to concentrate on her own life and growing up into a lovely adult.
My H left us in October for OW. We had a rocky start, I was a mess but we are coming through it well. I thought. I can go for a long time without getting upset infront of the DCs and try not to talk about him at all apart from when are you seeing him etc. Last night we exchanged angry texts and I got upset. my DD (14) got the gist of the texts and agreed he was being out of order. I thought that was the end but since found out she has text him later telling him to leave me alone, hes upsetting me and she has to pick up the pieces and hold things together! I have really tried not to lean on her emotionally and didnt realise she felt this way. How can I put this right and just let her carry on without worrying about me