You have made a mistake. 

 Let's say you have lent me $100, and now I am giving you lots of excuses why I can't keep my word and repay the debt to you.  Why did you trust me?  You know I'm a liar and have a bad reputation for not repaying my debts.  You should have known better.  (You should have known that you would never see your money again.)

You buy a car from Bob.  He swears it's in perfect condition and he'll be happy to return your money if you are not happy with the car.  A week later the car breaks down.  The mechanic wants $1500 to repair it.  You go to Bob, explain the problem, and ask for your money back.  Bob tells you that he's sorry but the car is yours now, and he denies he made any kind of promise to you about giving back your money if you were not happy with the car.  He adds that if the promise was not in writing, it wasn't given, it never happened. 

You should have known better.   (You should have known that you could not trust Bob..)