because it is not the things we did that we regr

It is not because things are difficult that we do

不是因为我们不敢做的事情很难，而是因为我们不敢做很难的事情

这句话的主句是后半句。it做的是形式主语，主语是we
it is not because...it is because....这里用的是这个句型，不是因为，而是因为。第一个that引导的是一个强调句。第二个that引导的是结果状语从句。

because it is not the things we did that we regret
的意思
because it is not the things we did that we regret

意思是：
因为我们很抱歉这个（事情，行为）（并）不是我们所为。
it is not the things we did that we regret
是：it is 。。。that。。。 强调句型
还原为：we regret (not) the things we did
则容易理解很多。