The best thing to do is always change the same amount of fiat into BTC when you make a BTC payment (or other crypto)

The best way to avoid this catastrophe is to just not spend cryptos. I don't . I just use them for investing only.

I fact I don't even spend that much money at all, my biggest spending so far was my house which I bought a year ago, and my recent wedding. But those were necessary.

Other than that I barely spend. I just keep it and grow it.