Why write music? Hasn't it all be done before?
It's funny, I hear this kind of question a lot. People think there's a finite number of notes and ways they can be put together. Or a finite number of colours that can be used in a painting, so you're never gonna come up with anything that hasn't been done before. In response I tend to ask them what hobbies they have and why they like doing them, surely it's all been done before?
And of course the answer to all these is "it's irrelevant". The difference in the art you choose to make is not in what you make, it's in what it inspires in others. I never get tired of seeing how my music affects others, because it's always different. And it rarely reflects what I felt when I created it.
So remember, art only becomes art when people experience it. If you're inspired to create, don't try to create something new. Just create what you feel, and let everyone else interpret it.