I think you're getting some good advice here. I may be able add a little something given that I originally identified as mono, and my spouse initiated the poly exploration in our relationship.
You may have known about your partner's poly desires 20 months ago, but you have only started to work with issues related to it over the past 2 months. That is a short time over which to change your world view!
My spouse and I had several bumps along the way-- she had a few affairs before we put our heads together to find that she was very poly oriented (17 years into our relationship!).
After I said 'yes' to living poly with her, she let me take a *long* time to get up to speed-- 6 years! Over that time, my comfort with poly grew. We would flirt with other people, she would tell me about her crushes (keeping them mostly platonic), she would encourage me to meet other women, and we even visited the 'orgy tent' at Burning Man a few times.
Only after I found a girlfriend did she feel comfortable embracing other relationships. And you know what? It's still emotionally hard for me sometimes. Since you are younger, it may be easier for you to change your orientation/lifestyle/whatever. BUT, you should still be allowed time to make the change, and it may have to happen incrementally.
Regarding 'neediness': In my case, most my neediness came out of an inability to communicate my emotions effectively. Until I was able to put my feelings into words, words with sincerity and strength, I could only get mopey and act needy. I think men in our culture typically don't develop very good skills at speaking their emotions, especially around things that make them feel vulnerable. Respect your emotions, and communicate them clearly so your partner will respect them too.
Wishing you the best with this. Post updates!