City were 3-0 ahead in Tuesday's Champions League on 70 minutes before Feyenoord's fight-back.
Guardiola said afterwards: "Difficult to swallow right now.
“The game was good, we scored three and we could have scored more. At the end we are not stable enough to do it.
“We did a good game but then we were not able to control the last minutes.
“We have to accept it for many circumstances. The team was so committed in many things.
“It happened today. We are not able to win games.
“As a team we have always found a way to win games. Right now, after 3-0, other situations, it’s as if nothing happened. We could not score the fourth or fifth.
“Unfortunately something happened and we were not strong enough and we have to avoid the mistakes.
“Everybody knows the situations. We have to prepare and recover. Have a day off.
“It has been, is being and will be a tough season for us. We have to accept it for many circumstances. The team was so committed in many things.
“Unfortunately something happened and we were not strong enough and we have to try to avoid the mistakes."