Ovulation App. And how to use it?

hey im trying to decide what i should do with my method of birth control. ive started the combination pull but it gave me reaallyyy bad side effects. so i am thinking about going back to the oldest form of BC (pulling out). me and my bf were doing that for 4 months until i started the pill. so i wanna go back to pull-out. can anyone recommend me which ovulation app i should use (i need one thats the MOST accurate!)