To me, a husband, whose wife has very recently asked me to allow her to seek a bf and is plenty anxious at the prospects of them being out on dates, I marvel at and admire that you've had a mostly postive introduction to your poly relationship.
In the beginning, it was fine. I was fine with them being alone. It was sweet.
Was this hard to achieve and was this complete or was it something you either told yourself or worked through to achive?
If it was either incomplete or something you convinced yourself of, I could see a more extreme period where they're alone having a grand time while you're feeling misrable letting negative feelings emerge that you may not have known were there.
FWIW, I usually have many demands on my time every day so tend to prize my alone / me time. I'll sometimes stay up very late on a Friday night after everyone has gone to sleep to really relax and do what I choose. That said, I find when my wife is out late travelling or overdue visiting with friends causes me to be unsettled and concerned until I know that she's back and okay. It may not make rationsal sense as I do very much enjoy my time with her, but is how I feel.
Best wishes to you in this,