How should i break up with my girlfriend?

I've been going out with this girl for about six months, and as i've gotten to know her i've realized she's not as mature as i thought she was. she's 13, and i'm 15, i've never done this before, and i really need to get our of this. how do i break up with her without making her feel like garbage.How should i break up with my girlfriend?
tell her the relationship is not working. Use the dreaded ';lets be friends: verseHow should i break up with my girlfriend?
Just say, ';hey, um is it okay if we can just stay friends for a while?';


or '; I think we need to take a break from eachother.';