Please help me..WAS IT ALL MY FAULT?

Well to see, it is your fault because you did not see the note which you gave properly. But on the other hand you were unknown of the fact that your 20 quid was in your purse itself. You can go and tell the owner that it was your fault and tell the owner to take him back. If you people don't know where does he live, god may help if it was not your fault.