Why is FOMO so hard to deal with?

Is it still ‘Fear of missing out’ if I actually AM missing out? I can’t go to my friends (A) country house over the long weekend because I’m working. This would be fine except that our other friend (B) is going, and I literally can’t stop thinking about it. I feel like I’m missing so much, like they’re gonna be with friend A’s family and bond and everything. I also feel like friend B already has a stronger bond with friend A than with me, but me and friend A have known each other for longer, so we also know each other better. I feel like I’m being so unfair because we’ve hung out before without friend B, because we have some activities that we specifically do together etc. It’s like I’m jealous of both of them and I am just so sad thinking about it. I feel like such a bad friend but when they were talking about maybe going I was secretly hoping that friend B would decide that they didn’t want/couldn’t go. And I’m also thinking about skipping easter dinner with my grandma and family because then I could go to the country house just a day later and spend the weekend with them. And then I’d have to leave a bit earlier than them. But I also don’t want to miss Easter, not as much because of FOMO but because it would be rude to cancel and I want to spend time with them too. I think I’ve always been a ‘jealous’ friend, but I never show it. How do I stop thinking like this ugh…