The Magpies suffered their fourth defeat in five matches across all competitions after Thierno Barry scored early in the second half to secure Everton's victory. Newcastle's frustration increased when midfielder Jacob Ramsey was sent off in the closing seconds for a foul on Harrison Armstrong.
Jaissle and his players were booed by supporters at the final whistle, with Newcastle now sitting 13th in the Premier League after winning just twice in their last seven matches.
"After a performance like that I can understand. That's the reality and I am responsible for that," Jaissle said.
"We need to work on the things we need to improve and after today that is a lot."
The defeat was Newcastle's second against Everton in five days following their EFL Cup fourth-round exit on Merseyside last week. Jaissle was unhappy with the decision to allow Barry's goal, believing Fabian Schar had been fouled in the build-up, but admitted his team had not done enough to deserve a result.
"Unfortunately, we couldn't deliver. Over the whole 90 minutes it was not our game and our style of playing," he said.
"We were not brave enough in possession, too many long balls. If you cannot deliver when it counts, then you don't deserve to win."
The 38-year-old also acknowledged that confidence is becoming a problem for both young and experienced players as Newcastle struggle to find consistency.
"There are certain moments with not too much confidence, not only with the young lads but also with the more experienced ones," he added.
"It's a difficult period. We also know you can only overcome that as a unit."
With speculation mounting over his future following his appointment as Eddie Howe's successor, Jaissle faces a crucial away fixture against Fulham on Saturday. Another poor result could increase the pressure on the Newcastle hierarchy to consider a managerial change during the international break, before the Magpies return to action against Premier League champions Arsenal.
Chelsea will be hoping to build on their victory over Manchester United when they travel to face Sunderland, although Xabi Alonso continues to contend with several significant injury concerns.
Kobbie Mainoo believes Michael Carrick has helped him become a more confident and influential midfielder at Manchester United, particularly by encouraging him to take greater responsibility on the pitch.
Hansi Flick has backed Raphinha, Gavi and Alejandro Balde to play important roles for Barcelona as they prepare to face Aston Villa in the Champions League on Tuesday.
Arsenal have received a significant boost with William Saliba reportedly completing his recovery from a back injury and returning to full training.
