What I have found is that when you are in a poly lifestyle and you add a new partner or partners, the time available for the primary partner is going to have to decrease. I like to have 4 nights to myself per week which allows 3 nights for my partners. If I add a partner, I am not going to decrease my alone time to fit them in. The time spent with the new partner will have to come out of the 3 nights and that will decrease the time available for my existing partners.
I guess you are feeling fear of abandonment since you guys have been fighting so much lately and he's probably experiencing NRE with the new person. I agree with everyone else.....find some new things to occupy yourself and maybe work on some of your issues which may contribute to the fighting that you guys have been doing. You know how that is.....I always carry my issues into every relationship until I do my work to resolve my part of the conflicts.