Antonio Conte's men went into the 2017-18 season as defending champions having won the title in record-breaking fashion last term, but with three games remaining they find themselves in fifth place and five points adrift of the top four.
Chelsea could still end the season with silverware having reached the FA Cup final - where they will face Manchester United - but Courtois acknowledged that their season could have been a lot better.
"I think there have been ups and downs. We played some very good football and had some very good victories. But there were moments of maybe two or three weeks when we didn't perform and we lost some points," he told the club's official website.
"When you start the season, there are six teams going for the title. This season, the top spot was out of reach, but the other positions were quite close. Then it is down to who is the most consistent and I think in some periods we were less consistent than the other teams.
"At one stage we were second and we could have stayed there, but we had a bad spell and dropped to fifth. We can't be happy with that, and also the League Cup we got to the semi-final and unfortunately lost to Arsenal. Overall, it's not a bad season, but we could have done better overall and I hope we can win the FA Cup."
Chelsea will face Liverpool at Stamford Bridge this afternoon realistically needing a win to keep their top-four hopes alive.