ok...now i know for a fact u should leave her. people like that only bring unwanted drama to your life. trust me.
Firstly, If your gunna break up with her you shouldnt tell her you cheated IMO, It won't do her any good, if you were planning to stay with her i'd say tell her, as honesty is needed in a relationship. but coz your breaking up with her anyway I dont think it will do anything but cause her extra pain. Secondly, It sounds a little bit to me like this girls putting a guilt trip on you so that you stay with her. I'm not saying im 100% right, thats just what it sounds like to me. Because you mentioned u were losin interest, she cries n tells u ur the first to make her cry in 4yrs, and then tells u about her dad and all that jazz. I might be completely wrong though i guess it depends on the girl and what shes like. And if you do break up with her, Definatleyyyyyy do it in person man, its just the right thing to do.
dude you are young. be single. seriously. there is too much pussy out there for you to be in a relationship now.