i wanted to tell me my mom everything that had happend on campus over the past few months

everything为补语(宾补),这里that引导定语从句,修饰everything.
tell的用法: tell+宾语+宾补(就是"宾语补足语"的简称),如果把上面的句子改成 "I wanted to tell me my mom that everything had happend on campus over the past few months."那还勉强可以叫补语从句(实际上它还是宾语从句,因为宾语补足语属于宾语的范畴)
这里的everything与that并不是一种特殊的搭配形式,定语从句中,当先行词是不定代词everything/all/anything/noting/someting等应该用that当关系词.
拆开可以说"I wanted to tell me my mom everything.It had happend on campus over the past few months"或者改为宾语从句"I wanted to tell me my mom what had happend on campus over the past few months."

这应该是个定语从句
先行词是everyting
that是关系代词
that had happend on campus over the past few months做everything的定语
everything和that不是固定搭配，但是当先行词是不定代词everything,all,noting，someting等应该用that当关系词

that在这里引导的是一个后置定语从句，修饰everything，这句话的意思是我想要告诉我妈妈过去几个月在学校里发生的一切事情。

是定语从句
理解为补语从句为什么不正确
因为我们学的英语和老师一开始就说这叫定语从句