The veteran is the LaLiga's leading goalscorer with 16 goals, but has hit the wall in recent games.
After defeat to Atletico Madrid last night, Lewandowski said: "In the last two or three games, we have had great chances that we have missed.
"I myself have had the feeling that I have lacked a bit of calm in the area, and perhaps confidence in myself."
The Pole also said: "I think the team is even angrier because we played a very good game in the first half. We were very good, we were even good in the second half, but because of our mistakes, Atlético created chances and scored two goals.
"We played good football, but no matter how you play, if you don't take advantage of your opportunities, it's difficult to score goals later on. You can shine, but we lacked the finishing touch and that explains the defeat.
"In the last few minutes, we played as if we were in a final and we were losing the match and we had to score at all costs. Sometimes you have to know that when you want to win but you can't, you definitely shouldn't lose the match."