How should i break it off?

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This guy i've been dating "unofficially" for about 4 months has been kinda "blowing me off" for the last month and is just really pissing me off and i'm sick of his crap. How should i end this whole thing? He keeps telling me to not leave because he's just busy but he's not because he's going out with his friends every night. (he's 21 i'm 17) So please help