How to tell an ex you are dating someone else?

Back in December my girlfriend and i broke up. over the past few months she has begun speaking to me again and hoping to get back together. I have remained distant but polite. Just a week or so ago i met another girl and we have ended up going out and i am very happy with her. My ex has been messaging me asking why i have been so quiet and i am wondering how to break the news that i am dating someone else? she is fairly unstable and i am worried about how she will react. I dont want to continue contact with her as i find any interaction with an ex a sort of breach of trust with my current girlfriend