Inter were thumped 5-0 by PSG in Munich, leaving the club floored after also blowing the Serie A title and being beaten in the semifinals of the Coppa Italia.
Sommer told BILD: "Lately we have been missing something in the important moments.
"Both Saturday, but also in the league, where we have dropped a lot of points. That's football.
"Everything can change radically in the space of two or three weeks. We have been punished harshly for our mistakes this year.
"That hurts us."