So this is a bit complex. I have had polyamorous relationships and was poly for many, many years. I started dating my fiance a year and a half ago. I told myself I wasn't feeling the poly life anymore and that monogamy was better for me. That I was using it as an escape from the abusive ex. I don't feel that's true anymore.
My partner is a dream come true and I adore them. I don't want to lose them. We are getting married in October and I just don't know what to do. They told me if I were to ever want to be poly that would be the end of us. Knowing that I used to be poly, so that scares me. I don't want to lose this great love but I don't want to miss out on more chances for love and exploring myself/life.
I am at a loss, anything helps.

My heart breaks for you, sincerely. I really wish I could offer you better support than this.

But your relationship orientation is part of who you are. Are you really sure you want to get married to someone who cannot accept-- or as far as it sounds, actively rejects-- something that is a part of who you are and the kinds of life experiences that you don't want to miss out on?

I am someone who now sees polyamory as a facet of my identity, rather than more of a choice. I wound up asking myself similar questions about the monogamous relationship I was in prior to coming out, but I chose to have the hard conversations with them because I want my closest relationships to be a place where my true self is fully, unconditionally accepted. It was exceptionally wretched to go through the realization that the person I love, and was invested in building a life with, wasn't up to living in a way that allowed me to support that side of myself. But I think I would have felt even worse trying to repress my emotional experience and pretend I was someone I'm not for the rest of my life.

So I deeply feel for what you're going through here. I understand the fear and grief of believing that you have to choose between being yourself and having a great love present in your life. In your shoes, I would probably even delay the wedding at minimum, but I'll fully admit I've never been engaged or planned a wedding like that, so take that aspect of my advice with a heavy dose of salt. I just have doubts that the two of you will have this matter comfortably figured out by October; as far as I've heard, simply making a wedding happen can be hella stressful.

If you aren't already doing so with your partner, you could seek out a couples therapist who specializes in ENM/polyamory to see if there's a way to salvage things. Doing that together allowed me and my then-partner to understand each others' relationship needs to a degree that we otherwise would not have been equipped to handle on our own. We now have a very strong supportive friendship, and I wholly attribute its existence to the decision to see a couple's therapist together as we decided what we wanted to do with our lives.

I truly hope that your partner is the kind of person who can see the value of what they have in front of them, and is willing to grow in a way that supports the person you are rather than only the style of relationship they want for themselves. Hang in there