Despite dominating for long periods, the Lions were held off by a resolute Brighton defence and Adam Virgo's header after 72 minutes earned the Seagulls a priceless three points.
Jackett said: "I thought in the first half we played well and we should have been ahead because we had a couple of good chances.
"But in the second half we couldn't find the energy. We've got a couple of players out injured and the lads we brought in are maybe lacking a bit of fitness.
"Obviously we need to put in the performance for the full 90 minutes. I was looking for the players to take the game to Brighton after the break but we didn't create many chances."