Barcelona goalkeeper Ter Stegen was on the bench for both of Germany's UEFA Euro 2020 qualifiers during the international break.
Manuel Neuer conceded four goals in a Group C defeat at home to Netherlands, but then kept a clean sheet in an away win against joint-leader Northern Ireland.
Ter Stegen deputised for Neuer during the 2018-2019 season when the Bayern Munich captain was sidelined long-term due to injury.
Neuer has regained the gloves since returning to full fitness, though, and Ter Stegen accepts he is in a frustrating situation.
"It is not easy to find an explanation as I am giving the best account of myself in each game to make the decision more difficult," Ter Stegen said.
"I am trying [to become first-choice] anyway. But this trip with the national team has been a tough blow for me."
Ter Stegen, 27, has played 22 times for his country, who are on track to qualify for Euro 2020 after joining Northern Ireland on 12 points after five games.