How do I (23F) deal with my boyfriend’s (21M) best friend (21M) constantly stirring drama in our relationship?

I’ve (23F) been with my boyfriend (21M) for 2 years now. From the beginning his best friend (let’s call him Sam) has always been seemingly bitter and weird about us. 

When we first started dating, Sam told my boyfriend I’d cheated on my ex, which wasn’t true — my ex actually cheated on me (we also broke up 5 years ago, so not sure why it’s even being brought up). I brushed it off as I thought maybe this was just a rumour he had heard and he thought he was looking out for my boyfriend. But a few months later, he told my boyfriend another lie about me cheating on my ex on a family holiday - again, this just isn’t true, i never even went on a family holiday with my ex.

Then a few months ago, I had a quick 5 minute conversation at our local bar with my sisters (male) friend who I’ve known for 10 years about my sister’s wedding. Sam saw us talking and told my boyfriend to “watch out,” making out as if something inappropriate was happening.

Last weekend, I went out with my boyfriend, Sam, and their other friends. Sam brought a friend none of us knew (“Jack”). Jack and I didn’t speak all night, but right at the end he started chatting to me about travelling - nothing flirty, just a normal conversation. Out of nowhere he tried to hold my hand. I pulled away immediately and went straight to my boyfriend to tell him.

My boyfriend confronted him, but Sam took Jack’s side and they accused me of lying. It was honestly humiliating being made out to be a liar in front of my boyfriends friends, and to make it worse his ex was also there. Now I just feel embarrassed, like people might think I made it up for attention.

My boyfriend even jokes that Sam is a pathological liar, so I guess this behaviour isn’t surprising. He also constantly invites my boyfriend’s ex out with them but never includes me unless my boyfriend asks. He gets annoyed if my boyfriend leaves their PlayStation party to see me and has tried to convince him not to meet up with me multiple times, even though we only see each other 3 times a week.

I’ve genuinely done nothing to this guy, but he clearly has an issue with me. I think it’s a mix of him being misogynistic (he hates 90% of women) and is weirdly clingy with my boyfriend. These lies he’s told my boyfriend have never caused issues for us since my bf knows he’s a liar and loves drama and he trusts me, but it’s still so frustrating.

What’s really bothering me is that my boyfriend is still so close to someone like this. I’m not going to tell him to cut off his best friend, because they’ve been best friends for like 10 years now. but it honestly feels like a slap in the face that his best friend treats me this way and nothing changes. My boyfriend says he has my back, but it’s frustrating it’s still happening. I don’t know what I’m supposed to do about it. Any advice would be appreciated thank you!

TL;DR: My boyfriend’s best friend keeps lying about me, twisting situations, and trying to interfere in our relationship. It’s causing constant drama, and I don’t know how to handle it or what boundaries should be in place