On the one hand you obviously regret your wrong-doing, which is good since it shows you have learnt your lesson and will not offend again.
On the other hand, you are over-thinking it all. I know it's easier said than done, but take a deep breath and try to put it to one side for the time being. You have to be patient and wait for the letter in 3 - 4 weeks; there is nothing else you can do, apart from save up. Everyone has said don't worry about the Oyster card - it is very, very, very highly unlikely that they know anything about it, will not have any triggers to alert them to it, and will have no evidence to try to make you pay for it.