So if someone loses a considerable amount of money, how can they just shake it off?

Let’s say someone loses their month’s worth of income. Now by all means, if there is a possibility of getting it back do it. If not, the first thing you do is to learn not to make that mistake again. Beyond that though if you let it bother you, it will start affecting your health and your relationships. So it certainly does not make sense to let that keep bothering you.