I'm 3 months into my relationship. I was adamant about being mono in the beginning and not dating anyone poly. However, that was the only way I knew to relate. Being mono was unchallenged before then, even though I was quite convinced that I am/was.
My partner is in established relationships with 2 OSO's. What amazed me was his ability to be fully present with me even when he was in the midst of everyone. I had never had someone be so focused and clear in all of my monogamous dealings.
The other thing that I questioned was the root of my needing monogamy. Did I really think that someone only had enough love for one person?
I believe (or thought I believed) in an abundant, unlimited Universe, full of all the resources that I desire. That includes love. There is no cap on it. It just flows. To me, I was paying lip service to that belief. This relationship is forcing me to live it. I experiment with my life so this is a great thing. I'm even considering being poly myself.