It was about a year ago when I became interested in my (now-ex) boyfriend's best friend. I was feeling torn between having to choose between them and told them that I wanted to date both of them. A big mess happened, but I decided to stay with my boyfriend and just be friends (with emotional attachment) to his best friend. I knew that I was poly then, but I thought I could suppress it. Now, I don't know.
Funny thing is, something extremely similar happened when I had just started my first relationship when I was 15. How I got there was much more convoluted, but basically I was also crushing hard on my then-boyfriend's best friend and I wished that I didn't have to choose. For a time felt that I didn't have to, but then I saw how much it was hurting him, kicked myself mentally and decided that I had a previous commitment to my (then) boyfriend, and to stop being immature and insensitive towards him.