My girlfriend's friend is trying to ruin our relationship. What do I do?

USAF_Fighter
There's this girl. She used to be my friend until I realized how much she pissed me off and now I really don't like her that much. Anyway, my girlfriend is her best friend. The friend, let's call her Lin, doesn't think I don't like her because she thinks I'm joking when I act like it. But ever since I started dating my girlfriend, she's been getting in between us and trying to ruin our relationship. Like she's extremely jealous of me and wants my gf all to herself. By the way, I'm in high school because that will probably make a difference. So on the first day of school, I'm walking to my locker with my girlfriend. Lin grabs her by the wrist and pulls her down the hallway. Keep in mind I hadn't seen my girlfriend in a week and a half. Another time, I was at the lake with my friends and a girl sat on my shoulder for a second jokingly long enough for someone to put it on snapchat. Lin screenshots this and sends it to my girlfriend, who cries all night and blames me. And Lin is always trying to get my gf to chose one of us. Like she'll tell my gf to hit me and sees if she does it (she doesn't). And Lin is always pulling my gf away to have private conversations. Most likely trying to get my gf to hate me. And whenever I am next to my gf or I hug her, I get the "I'm going to break your @ss up) look. I want to tell my gf this, but I feel like it would break her heart. I don't want my gf to get upset, but Lin has been such a b*tch lately. I love my girlfriend, but I don't want this to continue. What should I do?
My girlfriend's friend is trying to ruin our relationship. What do I do?
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