It is surprisingly difficult to answer this question without naming the obvious: love, friendship, nature, art. As always, the simplest questions are the most difficult to answer.
So, let me try to be more specific. What gives me peace? My violin. Practice. Losing myself in the vastness of a great musical imagination, exploring a fugue by Bach, a sonata by Brahms, trying to do justice not only to their technical demands, but to the many shades and tone colours, their architecture and their emotional depth, trying to communicate these things and to touch people without being imprisoned by words, by custom, class, or dogma.
Music - all art, really - can take the chaos and injustice of our everyday experience and give them structure, meaning, purpose. It may be an illusion, but if so it is a necessary one. Deep and violent emotions become part of a larger whole, of a journey, a transformation of despair, rage, grief and sadness into a kind of beauty, a kind of joy.
Most of the time I am setting myself up to fail. My technique is not adequate, my concentration wavers, my musical insight is too limited. But there are moments when I forget all this, when everything magically joins together, when I forget that I have a violin in my hand and that I am playing from music written long ago. Then I begin to sing. The cruelty and triviality and fragmentation of the world we live in recede and I find myself at once deeply alone and profoundly connected. In these moments the instrument allows me to utter a few halting words in the language that unifies all humanity. I am soaring, I am myself, I am at peace.