If you don't make an effort now, you'll certainly regret it when it's too late.
Tips
history
From Fan Ye's Southern Song text 《后汉书·光武帝纪》: "反水不收，后悔不及" — spilled water can't be gathered up; regret comes too late. Emperor Guangwu used this as a warning. 后悔莫及 is the standardized later form.
usage
Typically used in warnings: "if you don't X now, you'll 后悔莫及." Rarely used for completed regrets — more often for consequences still looming. 莫 here is classical for 不能 (cannot).
