Tottenham led until the closing stages of the first half, beforePedro cancelled out Christian Eriksen's opener, and Victor Moses netted the winner for the Blues.
Spurs now have one win in 10 games in all competitions and the week which saw them eliminated from the Champions League concluded with yet another loss at Chelsea.
"I thought we were fantastic for 44 minutes and then it was very disappointing to concede the goal before half-time. I think that changed things a lot," Dier told Spurs TV.
"That's where we need to have a strong mentality and not let it change the way we play. They had a lot of confidence after that.
"We showed fantastic character to start the way we did after what's happened in recent weeks and the Champions League.
"It's a matter of us playing like that for 90 minutes. It's turning lots of draws that we have into wins and seeing out results. That's massive for us.
"We work very hard, we're committed, we've got a lot of character - we showed that last week at West Ham and first half here - but we need to be more consistent."
Dier lamented the concession of "two sloppy goals" which saw Chelsea inflict Spurs' first Premier League defeat of the season.
"If you look at the bigger picture, this is our first loss," Dier added.
"Obviously we don't want to lose against anyone, a lot less Chelsea, but it's done with now.
"I don't think we can kick ourselves too much. We've got to be ready for Swansea next week, keep pushing and keep trying to improve and reach the levels we know we can."