If you only think she isn't interested then do as others suggest and talk. Find out if it's true and if so whether anything can be done to reinvigorate the relationship.
On the other hand if you have already split up whilst talking may make you feel as if you are trying to achieve something, folk rarely change their mind. They steel themselves up to make a break they've been feeling is needed for a while, and once they find the courage to do so they tend to want not to go back on it. In which case you will need to come to terms with it.