Unraveller by Frances Hardinge

In a world where anyone can create life-destroying curses, only one person has the power to unravel them. Kellen does not fully understand his talent, but uses it to help those who have been cursed, including his ally and closest friend, Nettle. But Kellen himself is cursed, and unless he and Nettle can release him, he is in danger of unravelling everything - and everyone - around him.