Two goals in two second-half minutes from Danny Green and Solomon Taiwo hauled the Daggers level but they conceded a late winner to Scott McGleish.
Harris said: "We caused our own problems in the first half and certainly at this level we cannot afford to give good teams like Orient two-goal starts. We worked hard to get back on level terms and then conceded again which is very disappointing.
"When we equalised, I thought we could possibly have gone on and won the game. But all the damage was done in the first half, we can't give goals away like we did there.
"This season is a major learning curve for the players at this club but what we have to do is to go back to the club and look at the DVD of the first half, reflect and learn from that. We can take a lot of positives from the second half and hopefully take those plus points into our game with Oldham on Saturday."
Source: PA
Source: PA