假如你叫betty你写一篇短文告诉你的好朋友mary你发现你同学对你不好生日聚会没有邀请我你感到很烦恼

Dear mary,
I have a problem,my classmates is unfriendly to me.And they were holdning a birthday party and they didn’t invite me.Now I'm very upset and I can’t think what i did wrong ,idon’t konw why and i don’t what to do!would you mind giving me some advice?
yours,
betty