I try. I always try. As long as there are objectives on the board and you have models, there's always the chance that you're simply better regardless of army.
Yes, there are losing matchups, there always will be. I recently broughy my Ironjawz to a GT with a CP-farm build for max WAAAGH! and it went 1-4 - as expected. The one victory was against the Freeguild gunline that went 0-5. My games against Skaven and Deepkin? Never stood a chance. Against Fyreslayers and Tzeentch? Even matches where player skill and army composition mattered.
The point is that Age of Sigmar is an objectives game. If I had foregone the cabbage and put 30 Brutes and 60 Ardboys on the table, I would have killed less and scored more. Not doing that is on me. Of course there's inevitable frustration playing your army because there are always better players, better armies, bad matchups, bad scenarios. If it's a tournament, just finish the game politely and move on, if it's a friendly game, concede or start over.
But never give up until you're mathematically eliminated from the game. That's a recipe for frustration.