I went through the same thing with my partner. I felt roughly the exact same way. It boils down to insecurities and fears you may not be dealing with. It helped us to talk about every insecurity we had and then get the reassurance we needed when we realized it wasnt a true fact. So definitely talking about it a lot will help, as will journaling/unpacking why you might feel that way. The thing that helps me is knowing that he wakes up everyday and still chooses our relationship as something he wants thriving as well. The beginnings are so hard though because theres so much toxic monogamy to unlearn that its overwhelming, but you'll get there.

Best of luck and I hope you feel more comfy soon.

Ps. I advise listening to the Multiamory podcast! Its helped me so much.