My wife is different with her girlfriend than with me.
For perspective: this is actually one of the really wonderful things about having an open relationship - you get to find, explore, and connect different parts of yourself with others. People are so complex and multi-layered, it's impossible for any one person to be "everything". Just as you find different things you enjoy and find fulfillment with in your various friendships, these sexual/romantic relationships also develop their own dynamics and reveal unique flavors that make them special, satisfying, and important.
Your wife is revelling in different facets of her own personality and sexuality - things she needs another person who's not you to bring about and share with her. This is the big gift of non-monogamy: it's not "you can have sex with others;" it's "you have the freedom to go see all the things there are to find in yourself." It may help you to think about it like this: you're not sharing your wife with someone - your wife is sharing herself with you.
I can't help but feel a little trapped. I know I said this in the original post but things escalated to seriousness way faster than I expected/predicted. I don't know at all what I expected but it's not purely casual...
This is why I always tell people to leave their expectations at the door, and don't make promises you may not be able to keep. I made all those mistakes myself. You can't put parameters around people's feelings, you just can't. Well, you can...but they quickly fall by the wayside because the heart will wander where it's called. You can't know what will come - but you can know that you're capable of handling it. You can know you'll survive it. You can know you'll do your best. You can trust in all of that...your own strength, your resiliency, your convictions. And you can decide that the potential risks are worth the rewards.
Open relationships aren't easy. They poke their fingers in all your soft spots, dredge up all kinds of messy, unhappy feelings and worries. They are unpredictable. They bring things to your door that you'd rather not see or deal with..things you didn't expect and don't have a plan for. This is the bad part. The good part is how they double the happiness and love in your life. They connect you to other people and enrich you in ways you didn't imagine. They reveal parts of you you didn't know you had, or had long forgotten about. They make things more confusing and much clearer all at the same time. They're a rollercoaster sometimes...and sometimes they're a cold beer on a hot day.
It can't be easy, sitting on the outside of that and look in. I'm sure you feel excluded, bewildered, overwhelmed...take deep breaths. Be patient with yourself. You have thrown your map away and decided to forge your own trail. There will be disorienting days like this...there will be many, many others that aren't as your new normal settles down into rhythms and patterns you can start to feel comfortable with.
No one knows what's coming, but if you and your wife keep holding hands and talking honestly...if you keep looking at the compass and agreeing on the directions you'll move in, you'll walk this wild, weird, wonderful path together.