I need advice on how to feel

One day my best friend tell me she’s going to meet this guy and she wants me to come with because he has a friend (you guys knows how that goes) eventually we start dating the guys we’d always ride together to go see them mainly because she had a car and I didn’t. Throughout me and my boyfriends whole relationship him and her would throw shots at each other sometimes playful and sometimes not. She’d always complain why don’t you stand up for me why do you always stick up for him. In my opinion I always treated them fair whoever was in the wrong so one and so forth I really didn’t want to get into it really because I was scared to lose my best friend and scared to lose my boyfriend. She made me feel bad about it all the time my best friend is bold so she really doesn’t care what she says and loves to put on a tough act. Skip forward three/four months me and my boyfriend broke up. They suddenly put aside all their differences and become “besties” always texting and her going to see him without me fast forward me and him get back together but it’s not the same he talks to her more than he talks to me she knows more about my boyfriend than I do and their seeing each other while I’m busy he even once told me I can talk to her but I can’t talk to you because you wouldn’t understand see maybe I’m overthinking and I trust my best friend and sh dated his cousin so it’d be extremely wrong for them to hook up but it’s only so close a female can get to your boyfriend without you stopping it she’s a flirt a tease Nd she’s bold Nd I’m not she asked me once if I didn’t want them being friends but I was scared to appear jealous or like I didn’t trust them