First, I'm very sorry to hear about your situation. You sound like you are in a lot of pain, which is understandable.
I'm not sure your history with poly but I wonder if Leaf has heard of the term New Relationship energy (NRE)? NRE is very powerful, that rush of feeling you get when you fall for someone. It is also ephemeral, so it is difficult to say you have a 'promising future' with someone you've only known for 1.5months. Typically at that point what you have is a whole lot of lust and NRE.
Was he always poly? If so, is this girl worth going mono for? Is there any way all 3 of you can sit down and talk about the capacity to love more than one person. It seems that in her NRE jealousy is ruling her feelings. I would suspect that as NRE fades he will feel resentful towards her for making him choose, but by that point he will have burned bridges with you.
Again, I'm so sorry for your pain.