How do you stop comparing your relationship to another partner’s dynamic?

My partner Ethan (36M) and I (36M) have been together for a while, and we’re in a consensually non-monogamous relationship. One of Ethan’s closest friends, Noah (36M), is also someone he has a sexual relationship with.
I’m realizing that what I’m struggling with isn’t really the fact that they’re having sex. It’s what I’m learning about Ethan through that relationship.
For some context, before me, Ethan had almost always been the bottom in his previous relationships. When we got together, he started exploring being more of a top with me, and that felt incredibly meaningful because I felt like I was part of him discovering a new side of himself.
Recently, Ethan shared that with Noah he’s much more submissive/bottom, and that Noah is the only person he’d really do that with. Noah is exclusively a top, so I understand why that dynamic exists. But hearing it affected me much more than I expected.
It made me realize that I’m grieving something I didn’t even know I was holding onto: the idea that I was the person who got to discover new parts of Ethan.
Now I’m wrestling with the fact that there are parts of him that only seem to emerge with someone else. Intellectually, I know that’s probably just part of loving another human being. Emotionally, though, it’s hard not to wonder what it means about my place in his life.
To make things harder, I’ve noticed my anxiety starting to show up in ways I don’t like. If I see a notification from Noah pop up on Ethan’s phone, I immediately feel my stomach drop. Sometimes I’ll ask who texted or what they said. There was even one evening where Ethan stepped into another room to reply to a message, and my mind immediately assumed the worst. He ended up showing me the conversation without me asking, and there was nothing concerning there. Looking back, I can see that my anxiety was writing a story before I had any evidence.
I don’t like that version of myself, and I don’t want to become someone who constantly needs reassurance or unintentionally puts my partner in a position where he feels he has to defend himself. I’m trying to work on that because I know it isn’t healthy for either of us.
The difficult part is that Ethan has actually been incredibly reassuring. He tells me he loves me, wants a future with me, and has even said that some of the confidence he’s gaining through these experiences helps him be a better partner for us. Rationally, I believe him.
Emotionally, though, I still find myself asking questions like:
How do you love your partner as a whole person when you know there are parts of them you’ll never personally experience?
How do you distinguish between your partner growing as a person and your fear of being left behind?
If you’ve been in a healthy non-monogamous relationship, how did you stop comparing your relationship to another dynamic when you inevitably catch glimpses of it?
How do you stop feeling like someone else’s connection with your partner somehow diminishes your own?
I’m not looking for people to tell me to leave him or that non-monogamy doesn’t work. I’m genuinely looking for insight from people who have navigated this kind of insecurity or have been on either side of it.
I love Ethan deeply, and more than anything, I want to learn how to love him as a whole person without letting my fear of being replaced overshadow what we have together.