<human>: Can you condense the text into three sentences or less?

I've had a great friendship with a female for several years. We actually knew each other as children, but developed a close friendship starting about six years ago. In the beginning, we had sex a few times, but decided that we worked best as friends and thus pursued other partners but remained most excellent friends. We have many mutual friends, and shared activities. 
My current partner and I have been dating for two years. We have had an incredible blossoming relationship, and we have supported each other through thick and thin since we started dating. She knew about my former sexual encounters with my best friend, but did not exhibit insecurities with the topic. Slowly, that changed, and finally, just a few days ago the three of us were going to meet and hang out at a concert. My partner exploded in a sudden burst of anger and verbally attacked me for over an hour. I had never seen her this way, and we cancelled plans to go to the concert venue. Now, she is forcing me to choose either my best friend or her without any compromise. I am bewildered, hurt, and (naturally) indecisive. She won't let the topic down, and any time I see her, she basically asks me, "well?"
<bot>: Partner knows sexual history with best friend which happened years ago. She wants me to break off my friendship with my best friend. If I don't do that, my partner will dump me.