How do i keep my hair long after getting bad split ends?

i'm 15 i'm part Irish part German and part native american, my hair is really oily and i wash it at least once every 3 days. my hair stops at the middle of my back i'm 4'11 and i use suave shampoo and suave naturals conditioner i have really bad split ends that go from the tips of my hair to almost to the middle, i wanna keep it long. i usually keep my hair in a pony tail so please help me any tips (besides cutting it) to help me keep my hair long and even grow longer thank u.