That is always a hard thing to do. Just don't break up over text no matter how bad you think it will be! Just tell her that you still want to be friends but you just don't want to be in a relationship or whatever your reason! Hope that helped! :)
"hey ummm I just wanted to say that your beautiful and cool but I just don't think it's working out very well...I'm sorry it's not you its me I just don't feel like we belong together." BAM! Oh and you might wanna give her chocolate! and don't do it over a text!!!