Hi and welcome.
I am not sure I understand your situation. You and your longterm partner jumped into polyamory by inviting and embracing sexually and romantically, one of her best platonic friends?
What is this like? Living together? Seeing each other how often? Sex/dates only between all 3 of you at once, or are separate sex and dates allowed?
Many couples new to poly decide to start out with inviting a 3rd to join them. This is usually a big mistake. Thinking sharing a partner will prevent jealousy is actually not the case.
Of course you're jealous. Your primary partner has a 5 year relationship with this woman. You're on the fringe. Also, the new gf has to build a romantic relationship with both of you at once? This is difficult to say the least.
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, 34), miss pixi's Dom for 3 years