It's often referred to on here as the spouse being meat and potatoes and the new person(or venture, or first step into poly or whatever else is new) being some new cuisine. You still love meat and potatoes, but that new cuisine has so many dishes you've never tried before.
I've had to deal with it a bit lately because I have to readjust to Karma seeing someone again. The more we live this life the more I think that while I understand and have no issue with poly, it's not for me. But I am okay with Karma being poly and know that I have to work through my issues as they come.
What may help is just jumping in. I created more anxiety for myself worrying about the what ifs and the future and how will I deal, that I paralyzed any progress. So I finaly said, go ahead. If you sleep with someone I just need a heads up that it's going to happen.
The first time he called I almost had an anxiety attack, but I was able to pull myself in and figure out where the anxiety came from, address it and be okay with it.
As for DADT, I'm a curious person by nature, so sometimes the curiosity gets the best of me and I find out things I'd rather not know. Karma and I have worked it to where I take the lead. I ask questions about the day, what they did, what they talked about, but as far as sex goes I ask if they did, I make sure it was protected and then I drop it. I've told myself this is a privacy thing as well as a reign for my curosity. I do not need to know what they do in the bedroom. She probably doesn't want me to know, any more than I would want someone to know about the details of my sex life.
Looking at it as respecting her privacy allows me to cut my curosity off, without feeling like I am missing something. Karma and I have always had an open friendship. I've always been one of the guys with his friends, locker room talk and all. So it's an adjustment to not ask. But I know my personal boundries and I know I will let my mind run away with "well he doesn't do that with me" and I don't want that. I don't want to diminish my sexual relationship or Cookies so I stop myself before I can.
And spend good quality time together. Karma and I fall into the day to day routine and then he goes out and does fun stuff with Cookie and I'm left going "well why the hell dont we do that?" Usualy we don't because we cant afford it or my health isn't good so I let him use our 'date' money for her.
That will be changing, I found a ton of things I can do even on bad health days, that we will both have fun, are free or cheap and close by. I don't need it all the time, but I need to be reminded as well as remind myself that we can still even 9 yrs later, do fun things together.
My biggest self help was to stop comparing. I am not her and she is not me and therefor our relationships will be different. Karma has feelings for us both, but that doesn't make one of us less than the other. He likes us both for being us. So when I'm feeling like we aren't spending quality time, I look for things Karma and I will enjoy, not just things to make time spent even. Does that make sense? I'm not sure how to put it into words. I just recognise when I am feeling a loss of connection with him, so I work on getting that back. It's about our relationship as its own seperate thing, not ours in comparison to theirs.
Maybe this will help your partner see things in a different light.