if you know the person won't tell then you need to just relax. It is very annoying and paranoid when you tell someone something and you are worried they are going to tell. However, if you are certain this person isn't going to say anything, then don't bring it back up to them. If you do they may think you are asking for them for help so avoid saying anything like "Don't tell anyone what I said" it is better to leave it alone. This is probably bothering you more than them. They are probably not even thinking about it any more. I hope this helps.