I'm in a situation that I used to think of as poly...tbh lately it feels like a harem. My guy has a gf, it's all good. But he's a traveling musician and has a LOT of other girls he sees (FWBs). I'm comfortable with just the idea, but then seeing his pics on FB actually gives me a twinge of jealousy. I try to relax and be ok with it, but he ignores my messages when he's with them yet when he's with me he's always messaging other girls. How can I chill out and not be bugged by this?
If the situation overall feels okay, but seeing his photos on Facebook bothers you, consider not looking at his photos on Facebook. You can "unfollow" or "hide" posts by him.
It sounds like he has a case of "vacation girlfriend," which is a phrase my therapist used with me and which is really helpful. When you're dating someone long-term, and you see them every day, time with them starts to feel less 'special.' They just sort of blend into your day to day life, where you're running errands, checking your phone, etc. With people you see less often, or whose relationship is less settled-in and secure, it feels more like "oh, I have an afternoon with Angleesa, I should plan ahead of time and get all my stuff done and not be on my phone during it."
There are pros and cons to both type of relationship, but it's important to be intentional and realize when someone is a "day to day" vs "vacation" partner. It sounds like when he's with you, he's still in his daily life, where errands exist, phones need to be checked, and it isn't this set-apart bubble of time to enjoy each other. It's perfectly fine for you to point this out and say "hey, when you're with me, you're often on your phone - can we set aside some time to just be present with each other? Can you do more to cultivate our relationship, even though I'm around more?"
If he argues, denies, or refuses, if he acts like you're being unreasonable, then he probably isn't great for you to date. If he's willing to acknowledge what's going on and re-engage in your relationship in the ways you're asking for, great!
It can also help sometimes to just find a distraction of your own - casual dating, more time with friends, hobbies, new creative projects - so that his flightiness is less of your problem.
Ultimately, if you continue to feel like there's a part of your relationship that you need to "chill out about" and it's just on you to relax and let him act how he wants and not feel any feelings or ask for anything he's not willing to give, then you should leave the relationship. But talk it out with him first and see if he's wiling and able to make some changes!