I get triggered by feeling left out, too. It's a big problem for me actually.

It's true, that it's okay for them to hang out and be friends. And sometimes you might not be there. But the thing is, like, if they start purposefully leaving you out vs. you just weren't there because you had other plans. So, like you're going with your mom, so it's okay that they hang out without you, because you have plans. But, if they start always leaving you out and never inviting you, that's when it's a problem.

An example is that my ex-friends, one weekend they all disappeared. I had no idea where they were I called all of their houses and finally found out they went camping. They were my only friends and my dad was always off with my my step-mom, so I was home alone all weekend. Well, when they got back I confronted them why wasn't I invited? Their response was "you wouldn't have had fun anyway." That's not reason to not invite someone. How do they know I wouldn't have had fun? So, that was beign left out on purprose, and now I know it's because they weren't my real friends.