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.

It's something I've learnt to deal with after watching a friend who is slightly better than me in some aspects of the game. Generally he just plays a more aggressive game and has "more balls" so to speak.

I guess you're having the same problem that I had.. whereby if there were two suited cards on the board, my opponent called a reasonable bet and then the 3rd suited card comes on the turn and they bet. Put simply, be prepared to lose a little bit at first by simply calling to find out how often people have the flush. It's not something you can put a number on, more of a gut feeling. You just get the feel for it after a while, not something you can learn from a book or be told.

Another almost identical problem was dealing with a paired board on the flop. Even if the board was like A33 and someone bet at me i'd fold AK thinking they had a 3. Just something you learn to deal with.

Of course you'll get smashed by the flush or trips occasionally but the amount of pots you pick up between those will outweight the beats.

if he bets alot it looks like hes bluffing or has a pair i would call the bet.

yep ... bet