I've been struggling with insecurity for months now. I've improved a little, but I'm still not out of the woods.
I not only met my partner's other boyfriend, and her husband, but I moved in and began to rent a room from her boyfriend. At first, I wasn't intimidated by him. I got to know him, and started to think that he was quite an impressive guy. So, getting to know him didn't help me.
I second the suggestion to read a bit on xeromag.com
I would also recommend reading the book Radical Honesty. I'm reading it now, and it is helping me, particularly his recommended list of questions that couples should ask each other to build and keep intimacy regarding sex and relationships. He even mentions that it doesn't matter if your relationship is mono or poly, that his questions still alleviate stress and build intimacy.
I need help with insecurity too, so I don't want to try and give any more advice at the moment.