But The Thought!

Train hard, fight easy.
That old military rule applies to polyamory. If you work through your concerns early and thoroughly, things progress with much less drama than if you bottle stuff up or smile through clenched teeth.
This often means acting as a kind of meteorologist to yourself: recognizing the incoming twinges and nags that are the storm seeds on your personal satellite map. Then communicating.

Here's an example. In the very early stages of my relationship with Terisa, Vee was honest enough to tell me that she found it hard (who can blame her?) to hear me finish a phone call to Terisa by saying "Okay, my love. See you." or "'Bye, darling." Even when I was just texting with T., Vee sometimes felt as though my new girlfriend and I were spending an inconsiderate amount of time together even when apart. Fair one, I thought, so I took it on the chin and tweaked my behavior. Vee in turn soon got used to the (now fewer) "loves" and "sweethearts" I addressed to Terisa in her presence.

Not long afterward, the situation had mutated to such an extent that Vee came to me more than once and said "Matt, I'm concerned that you haven't spent enough time with Terisa recently." And even: "Let me know what I can do to help you guys make that time."
I made the same offers when it came to Vee finding time with her boyfriend. And so on. And we meant it.

We are now all at a stage where the following sort of thing happens. Yesterday I was cuddling with Terisa on her bed in the half-light, pleasantly tired after a busy weekend, when one of her two primary chaps came in and whispered: "Just look at the cuteness!"

Having said all that, here's a bit of a kick in the ass. I do warn against falling too much in love with voicing your own specific insecurities too often. It might help you to tell your partner, who's out on a date, that you're feeling freakin' anxious, just as you did last time they had dinner with this person and the time before. But it probably doesn't help them enjoy their evening: plus, self-consciousness and resentment might result. So, recognize when and where to give people space. Whatever's going on in your imagination about what they might be getting up to, sometimes you just have to deal with it privately. Have a bath or a beer, go for a run, or tell your fears to the dog. Then go to bed and leave well enough alone for the night.

Put another way: if a partner of yours is out holding hands with someone else, try not to pressure them into figuratively holding your hand at the same time. If they've already demonstrated that they have a good grasp of your concerns, relax a little. I've found that this tends to work.