Wow, bdsm and poly all wrapped up in a first real relationship. Sounds like a tall order. It took me years to go through all those phases / experiences in my relationships, so be patient with yourself and don't expect to have all the answers. Cut yourself some slack!
It sounds like your partner is loving, patient, and understanding. Let him guide you through this. He knows you much better than any of us strangers on the internet, and it sounds like he truly has your best interests at heart. One of the most important ingredients in love and intimacy is vulnerability. Yes, it's scary and difficult, but the payout is worth the risk.
Don't forget that these other women in your / his life may also be a valuable resource. Chances are they've been through some of these challenges themselves.
If you've made it this far together, then he knows and accepts that you're an introvert, a wallflower. Try not to worry about what people in the wider social circle will think of you and how you'll fit in. You'll find your place at your own pace. In my 31 years of being an introvert, I have nevern once been criticized for being too quiet. I have, on the other hand, heard (and made) plenty of complaints about people who don't know when to STFU. So you're in good company.
Dale Carnegie, author of How to Win Friends and Influence People, really highlighted the value of being a good listener. Letting people talk about what they love is the best way to convince people that you're an excellent conversationalist. Trust me, they won't even notice how quiet you are, they'll just think of you as someone who shares their interests and is good at listening. I recommend reading that book, it has timeless insight on how people tick.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker