In Fallen London, can discarded cards come back later or are they lost forever?
And what about cards which you tried and failed?
Discarded cards do come back. New cards are randomly added to your deck of Opportunities, but you need to qualify for each Opportunity at the time it's added to your deck. If you fail or discard a card, and then lose the requirements for the card, then you will not see it again. Failing a card will sometimes cause you to lose a Quality required for the card (like Connected: Criminals), or you might (for example) gain so much Watchful that you no longer qualify.
But most of the time, you'll see particular cards again. Feel free to discard the Opportunities that don't interest you at the moment if you're looking for a particular Opportunity.
Source: I played this game for several months.