In Tie Fighter (DOS CD version), I've been stuck in one of the very last missions for many months, playing it on Hard difficulty.
Yesterday I finally gave up on it and set the difficulty to Easy, played through that mission, and set the difficulty back to Hard. Just so that I can finally finish the damn game. I dislike the mission because so much is also about luck, like whether the enemy TIE Defenders will target you when you fend off the enemy transports (one missile will kill you and end the mission).
By the way, to me it appears the GOG version has a rather serious bug, which is quite easy to resolve though. I noticed that some of my keyboard commands were not working in the game (like trying to transfer power from shields to lasers, or was it the opposite... also trying to set engine power (speed) to zero didn't seem to work either), but then I remembered that TIE Fighter required the keyboard to be set to US layout before all keys work.
So, I added "keyb us" to the DOSBox config file of the game (the autoexec.bat part, just before the game is launched), and now all the keyboard commands work.