I am no expert on investments, but have been in the financial services industry long enough to see lots of people lose money just because they panic once the market goes down. They pull out investments at the wrong time and therefore lose the growth they accumulated through time.
What goes down will go up. Do not panic when the market is down, but actually take advantage of it. Invest more, if able - as you said though, do not invest what you cannot afford to lose. 😉