She flirts with me even though I have a girlfriend

(Borrowing friends account) Hi I'm an 18 year old male and I've been in a relationship with my current girlfriend for over a year. There's this girl that's younger than me (she's 15) who I've known for ages as she's basically my neighbor. Our families are very close and I consider her my girl bestfriend as we've been close for so long. The only problem is she has a crush on me and doesn't make it a secret anymore (I've always known has had feelings for me for a couple of years). Admittedly, I have kissed her before multiple times as before I met my girlfriend as I was attracted to her. I still am as she is very beautiful and shapely for her age. This is a problem as she keeps coming onto me and keeps making sexual advances for me (even though I know she's a virgin in all aspects and would not normally behave like this with guys). Again I admit I am finding her hard to resist as I know we have chemistry, she is so hot (trust me, she looks like a mixture of Adriana Lima and Mila Kunis) and we look good to gether but I definitely do not want to cheat on my girlfriend and she is slightly too young although she will be starting sixthform this year. Last night she popped round my house and started subtly flirting with me, whilst my girlfriend and friends were there:o Luckily only I understood what she was doing but it really made me uncomfortable and on the edge and I could tell she was trying to make my girlfriend feel insecure and competing with her (Again- she's not like this usually but I think her infatuation with me is causing this). My friends also started commenting on how attractive she, in front of my girlfriend and asking about her and saying would 'hit that' (have sex with her). She has had two long term relationships but I've also noticed she seems to be attracted to older guys (Not that much older, only like a year or two or in my case three and her previous relationship was with a boy I know who is two years older than her and lives close to us both and that lasted for over a year without any sexual interaction as she is quite conservative) What should I do in terms of telling her to back off? I don't want to lose my close friend but it is putting my relationship at risk

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