all that was in my mind was being at home with my family这句话的结构是什么呢?是固定句式吗?

首先,这句话是错误的.
正确的表述是：all that in my mind was being at home with my family这是个主系表结构,也可以说主谓宾.跟it was a apple一个句型.
分析如下：
【all （that in my mind这一部分是主语all的定语,定语从句）这一部分是主语部分,主语是all 】was 【being at home with my family这是表语部分】