Not sure where to put friendship posts - Need advice

So, I used to be a really insecure girl. It happened because I was cheated on for the first time about a year ago (I was 22) so I thankfully made it through high school and college without that horrific experience. Long story short, it completely killed my self esteem and I never realized how difficult it was going to be to get it back. So, I start talking to a new guy finally. We click well. All of a sudden, my "best" friend is flirting with him and he allows it. No, we weren't official yet but the fact that after a breakup like that, the first real guy I like is attracted to my best friend, pissed me off. So obviously that didn't work out, right? Currently, a year later, I decided to date my best guy friend. We are PERFECT and I love him so much. He is so loyal yet my insecurities still convince me that he COULD cheat. It sucks because that's how toxic of a mindset that can occur when you've been shown in the past you weren't good enough. Long story short - me, my best friend, he and her boyfriend all went out to drink. She was ALL over my boyfriend, flirting and touching him, and he would smile and participate in conversation BUT he'd quickly walk to me, put his hands on me, start kissing me, even asked me to leave for a second to go to the bar to get a drink. He knew what was going on and I felt bad for her own boyfriend who clearly looked pissed and was silent all night. He's obviously a good guy but all because of her, I am fuming (at her, not him) and it's only adding to my insecurities. Deep down, I still see her as a threat because of how she acts that way, as if she needs to have a one up on me. I just don't know if this is the type of girl I need to be friends with or if it's truly my insecurities that want me to drop her. Thoughts?