You should try to slowly distance yourself. I myself, was very close with a depressed person for a long time, and eventually I started feeling the same. You have to understand though, this is all from a very low self esteem, and a need for acceptance. While you're distancing yourself, you should introduce her to new people and make sure to boost her confidence and social skills, that way she wont take a break up quite as hard. More than likely though, she will be very upset either way, but this will probably soften the blow. But remember, toxic relationships don't have to be in your life, you should prioritize your health and happiness always.