what if your friends exclude you ?

Today I found out that my friends are throwing a party. I had no idea about this. someone asked me if I was going and I was very confused then I heard that they invited  everyone but excluded me. I can't even explain how I feel. I'm crying, I feel alone, I feel like I don't matter at all. I've always noticed that when I'm in the group I'm the least important one , my presence doesn't make a difference.