I am sorry for your pain. Crying hysterically for days on end to the point you can't sleep, work, play... it sucks! BTDT years ago.
Your self esteem is low. You seem to feel inferior because you're a bit older and quieter than his other 2 interests. You're a "wallflower." Ever stop to think about how some people are drawn to quieter people? They can be peaceful to be around, and what they say can be pithier than people who are constantly yakking.
It can be hard to believe a poly person can care about/love you to pieces, want to keep you, and still be drawn to another, or others. But that is how poly people are. If he was mono and drawn to another, he'd leave you. But he's poly and can love several. He's still checking in with you and wanting you to express your feelings so he can comfort and reassure you. Let him in! Let him feel your pain and insecurity, while still owning these are your feelings to deal with, maybe with a little help.
In my case, my gf is quiet and shy, my bf is outgoing and extroverted, I love both of them.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags (poly, F, 61) loving miss pixi (poly, F, 39) since January 2009, living together since 2013
"Master," (mono, 36), miss pixi's Dom for 3 years