That's sad news. Poor Mirage, she was a little beauty. Budgies are infamous for hiding health issues, many until it's too late. In the wild it's in their interest to do this due to being a predated bird. As a caged bird, though, it means we often realise too late that something's up. What to do? Keep the happy memories of your time together close, and be proud that you gave her some quality time in her short life. Chin up.