It's not uncommon to feel the same way you did in the middle school lunchroom.
I am 60 years old but am still sensitive about my friends not including me in things. For example, I went to a concert with two of them; I was the driver. While we were together, they talked about trips they were going on together that didn't include me. I asked about one trip in particular and my friend said she was going with another friend in a camper: There was no more room for another person and that was that.
I reached out to them because in the past I had done things with them that my husband wasn't interested in doing. One of the women is pretty controlling though. In fact, I bailed out of a trip this summer because I didn't want to be her roommate. That may be why she doesn't want to include me in other trips. Should I keep reaching out or just forget it?
Regardless of age, it's natural to feel uncomfortable when your friends talk about getting together without you---almost as if you weren't there. It's not that they are obligated to invite you on every trip or include you in every outing, but they should be more sensitive to your feelings. If there is "no more room at the inn," they should avoid discussing their upcoming trip in your presence.
When you asked about going, even if there wasn't any space for you, it seems like your friend could have framed her reply more nicely, and even have said how sorry she was. (Those words might have even come out spontaneously if she really felt that way.)
I understand your wanting friends to travel with but these women don't sound like the right candidates. You bailed out last time for a reason. You need to adjust your expectations of these friends and not count on them exclusively. Is there another friend with whom you can travel?
Hope this helps.
P.S. This is the last day to vote for the About.com Friendship Awards. If my work or posts on Psychology Today have you think about your friendships, can you take a minute to vote: