My question is: when you call on a bet and hit your straight then the flush card comes and your opponent bets NOT to call. how can you fold a straight in the even that there are 3 hearts, diamonds, spades or clubs out on the table and you think that your opponents were drawing for something?
I lose a lot of money to situations like this. What should I do in situations like this, or better, how to avoid them?

This is not a simple deal, but as in most poker situations, you must be able to fold. Even if you haven’t had a hand many hands, just keep on folding.
YOU WILL WIN.