I suggest never retiring or quitting unless you just run out of time to finish the game. If you are truly worried about being demolished quickly, I suggest playing the adventurer save to give you a boost up in the early game.
I've seen people turn around what looked like a loss, and in general you learn more by losing than by winning...just take it from me, this month is my first non adventurer GOTM, I've only submitted two wins, and I've been playing them since this past January. Yet my last win was on Immortal, despite never winning before playing the GOTM above noble. Despite losing repeatedly, I learned, got better, and suddenly I won a game on a higher difficulty level than I've ever won at before in any Civ game.
I've seen people turn around what looked like a loss, and in general you learn more by losing than by winning...just take it from me, this month is my first non adventurer GOTM, I've only submitted two wins, and I've been playing them since this past January. Yet my last win was on Immortal, despite never winning before playing the GOTM above noble. Despite losing repeatedly, I learned, got better, and suddenly I won a game on a higher difficulty level than I've ever won at before in any Civ game.