What would you do if a person you considered for many years a friend let you down?

Anonymous
Would you talk to that friend or just move on and let it go, just to avoid confrontation between the 2?

I dont know whether to confront her or just let it go and ignore as Im not in taht age to start an argument like I was a young person, even if I will never know the reason why she dissapoint me.

This friend did not take me into account for her sister bday, I also know her sister and they and me were former classmates since elementary school, then in high school many many years ago and we as a group of friends always hang out together as classmates. When we finished high school and each of us took different paths, even if for some years we never heard from each other again or kept in touch,, at some point along the way we again regain contact and from there we started to hang out and socialize, I may say, not very often often but from time to time we manage to plan to go out. Individually I hanged out with the 2 sisters but also together, several times. I even attended the mass in honor of the dead husband of one of them (she is a widower) many years ago, and then like 5 years ago I attended the mass when their dad died. Even these 2 sister`s other siblings and mom know who I am caseu we were close in elementary and high school and then later when we were in our 30`s and 40`s. Anyway, yesterday I found out through Facebook ( photo) that there was a bday celebration in honor of this friend sister at some bar, as she turn 50. In the photo there were other former classmates from high school that I also know and I had manage to stay in touch for the last 4 years. In the photo there were even 2 guys (other former classmates) that they are not even part of the friends group that hang out often do not even but were invited. In the photo there were 3 ladies (not the sister) who int he last 2 years we see each other socially and hanged out, those 3 ladies were invited to this bday but not me.
Updates
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In fact me and 2 more ladies were not invited either, and we are supposedly friends of the bday lady sister, the bday lady and the 3 friends that were indeed invited. I mean we all know each other and hanged out at some point in different times. So my surprise was when I saw the photo and saw this celebration

I believe it was impolite and not correct not even taking me into account for this gathering , they did not invite other 2 that they all know as well. I mean this friend let me down.
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I dont know if the bday lady, is mad at me because she stopped contacting me like 3 years ago or why she did not invite the others as well. I do know that the bday lady had been acting weird several years ago and she got distant from the other former friends who were invited to her bday, (go figure!!) but the bday lady was closer to me when we were in high school than to the others, even when we were in our 30`s or early 40`s, this lady hanged out more with me than to the other ones.

What would you do if a person you considered for many years a friend let you down?
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