How can i end a relationship that needs to be over if i just can't do it?

I'm a junior in High school and have been with the same girl for over a year and have never once gone 3 days with out fightin... we have no trust....no confidence or anything but wen were happy we are real happy but the fighting we have tried to stop but just won't....how can i do it if every time i try to i think off goodtimes