Stamina comes with practice, and with experience you will learn the "CUES" of when you are getting tired and you then monitor yourself to prevent accidents. It sounds a bit odd I know, but you learn to see the first link in the chain and then slow down to make sure you don't rush into any mistakes.
For an insight into how to be a proffessional go to pilotpsy dot com, it really turned my career around and made me a MUCH better North sea captain.
Good luck, chin up and always do you best
Si