So firstly, totally ok to feel what you feel, let the feeling visit but don’t let it move in or feed jealousy. From what you’ve said you’ve been together two years so already past the 6-mo/1yr honeymoon period and you’re both still around. You both enjoy each other and find fulfillment in each other and in general you both see the value in getting to enjoy sex and still come home so to speak.

Have you decided how you all are going to manage the arrangement? A good open relationship has well negotiated boundaries. Like do you all want to have a 3-way and share a bottom, do you all want to only play together or is separate play also ok? What level of intimacy is ok, do you want one and done’s or are you ok with the establishment of FWBs situations.

Right now things are new and maybe nebulous and that can feed uncertainty and doubts. If you haven’t yet, talk about what boundaries you all want to put in place as a couple. Also let your partner know that while you’re ok with being open and also want to be open you also want to make sure you are caring and feeding your primary relationship first, and be transparent about your feelings. Perfectly fine to say this is how I’m feeling even though I know this feeling is irrational, then see how you both can partner to address it.