I wish I had all the answers, but a simple first step is to hold her, take a warm wet wash cloth or paper towel, and hold it over her nostrils and soak them and wipe them, it could be as simple as a bit of bedding or seed or something lodged in there
also hold her up to your ear and see if you can hear wheezing or anything in her lungs which could indicate respiratory infection which would need antibiotic treatment
unfortunately, mouth breathing in general can be a sign of distress in birds and can be a sign of other illness even if it's not respiratory and just an overall sign of stress
once you can narrow down why she might be breathing this way hopefully we can help you find a solution to help her