promise that one day everything's going to be better for

这是一个宾语从句
that one day everything's going to be better for you都是promise保证的内容
后面还用到了一个be going to 句型,表示“将会”
be better for sb. 对某人来说变得更好
所以整句为：承诺有一天万事更加如意.^_^