Last season's UEFA Champions League runners-up are currently 12 points adrift of German top-flight leaders Bayern Munich and five behind Bayer Leverkusen as the season reaches the midway point.
And Lewandowski, who has scored 11 league goals this term, feels Dortmund have nobody to blame but themselves for not keeping pace with the leading duo.
"I'm not at all happy," he told Bild. "Despite scoring 11, I should have had far more.
"We've just given away too many points - not because we've played badly, but because we've wasted our chances.
"We've all got to blame ourselves, and it's frustrating.
"We all know that we need wins and desperately need to put a run together now.
"We want to start off against Hertha Berlin (on Saturday). We need to hit the ground running in the second half of the campaign to reduce the deficit on Leverkusen and Bayern.
"We cannot afford to drop any more points."
