I don't think I can be poly

My partner is poly, and I - at best ENM but even then. I'm not sure - got involved with them over a year ago when they were seeing other people as their primary. I was happy being just in the mix, casually, but in the time since I have become their primary.

We developed actual feelings for each other, and not long later they met someone else. I've been trying to be a better partner, but it's a struggle when they spend time together.

We have a lot of love for each other and they even said they would like to spend their life with me, but I don't know what to do. I don't know if I can spend my life like this, but I also don't want to split this up..

Is this doomed?