I've been actively polyamorous for the last eleven years, so I hope I can be of help.

Poly/mono relationships can be difficult. I think a lot of people (especially the monogamous ones in the relationship) experience at least some of the feelings you are currently experiencing. I am glad you recognize she loves you. A lot of people (myself included early on in my poly experience) don't have that as a solid belief and it sours things in the relationship, sometimes even resulting in the end of it.

Polyamory takes a LOT of communication to work successfully, as Lynds said. Sometimes we have to have difficult conversations with one or more of our partners or our metamours and that can be hard. But in the end everyone comes to a better understanding of one another and it means things can be better going forward in the relationship.

To answer your question, I practice non-hierarchical polyamory, meaning I don't have primaries, secondaries, or tertiaries. Everyone just fits in where they do. For me, I experience different levels of involvement. I have serious romantic relationships that may involve cohabitation, but at the same time I also have casual sex (with my partners' knowledge, of course). At one point I had three romantic relationships, which was honestly a bit much. I've learned I do better with two relationships and either lots of casual sex or a couple of friends with benefits.

Love in polyamory is a beautiful thing. I came into it when I fell in love with two people at once, many years ago, and realized that it was, in fact, possible to have lots of love for more than just one person. It's so vast and it gets shared around and it just feels beautiful. Compersion is one of the best feelings in the world, in my opinion, which helps lead to my favorite part of polyamory: building a family of choice. We all care for each other and look out for each other. It's wonderful.

Try and remember that, oftentimes, people look at love as a pie: there's a finite amount of pieces that get passed around and once they're gone that's it. But in truth, love is an infinite, untapped source. The restrictions in polyamory, I've found, come less from love and more from time and resources available. The love is always there, though.

I hope this helped. Feel free to PM me if you have any more questions.