what is not mine is not mine

你这个说法比较中国化,一般不这么说
英语里有专门的翻译：“命里有时终须有,命里无时莫强求”
What is meant to be mine,will be mine one day; what is not,will not.
=====
如果单纯是要表达“不是我的就不是我的”这句话的话,那就是：
it doesnt belong to me,it would never belong to me