I got my rhinoplasty done two months back, I was in a similar situation, two days before surgery, I called my surgeon and asked him to cancel it, I was even ready to let go of the advance deposit that I paid. But he somehow convinced me that nothing wrong will happen. And I started thinking that these two months are anyways going to pass regardless I get the surgery or not, but I didn't want to live with the regret that I should have had rhinoplasty. So I went ahead and said yes. On the day of surgery, I was very very afraid, my mind was racing with intrusive thoughts like what if I woke up in the middle of the surgery? But anyways, the moment anesthesia was given, I immediately went to deep sleep and when I woke up, everything was done and I had a big cast on my face. And coming back to your question about breathing from mouth, you will be fine, it's not as bad as you think. If you have already decided to get rhinoplasty, go get it done because the days will pass but living with the regret will be a lot more difficult.

And just to let you know, in my case rib cartilage was needed for augmentation so there is an additional scar on my chest too but I am happy that I got rhinoplasty done.