How did 90% of the posters completely miss the point of the OP’s question? Admittedly, the title is a bit misleading, but the OP’s actual question has nothing to do with the issur of kol isha; it has to do with an issue facing vocally talented men and women alike, the issue of sincerity in music, which is no small matter for those concerned with connecting to G-d (for themselves and others) through music. Music is extremely powerful, and the feelings a performer has when performing are often felt by the listeners and can make a huge impact on them, as well.
There is a certain awesome connection one can attain with G-d through music which this girl has likely experienced in the past. Music can be a form of tefillah, and some people connect to G-d through music as some do through limud Torah and as others do through gemilas chesed.
It is natural that while performing for others it is much harder to attain this connection, as it is almost impossible not to think about how others hear your music, and you are also much more focused on playing/singing everything correctly than you would be at say, a kumzitz.
That being said, if the music itself is music that is spiritual and meaningful, and has a good message, imho if you are AWARE of that and would like the positive message to come across and to inspire people, then it WILL come across (regardless of your exact thoughts DURING the performance). Thoughts of ga’avah are natural, but your awareness of and concern about them in and of themselves in a way prove that showing off is NOT your intent. It is highly important to keep that awareness, but as long as the thing you WANT to come across is that feeling of inspiration and spirituality, imho that is what will come across.