Florian Kohfeldt said he was "brutally empty" after Werder Bremen were left on the brink of Bundesliga relegation by a 3-1 defeat at Mainz on Saturday.
Werder failed to make the most of 16th-placed Fortuna Dusseldorf dropping points at home to Augsburg, who, along with Mainz and Cologne, secured their safety on the penultimate weekend.
Robin Quaison put the hosts ahead at the Opel Arena after Josh Sargent and Theodor Gebre Selassie failed to communicate and clear Karim Onisiwo's goal-bound header sufficiently, with Danny Latza teeing up Jean-Paul Boetius for their second after a brilliant run five minutes later.
Yuya Osako pulled one back for Werder before the hour mark but Edimilson Fernandes put the game to bed in the 85th minute.
Kohfeldt's side must now defeat Cologne next Saturday to have any hope of retaining their top-flight status, with Dusseldorf two points ahead of them in the relegation play-off spot.
Four-time Bundesliga champions Werder have only been demoted from the division once, spending the 1980-81 season in the second tier.
Kohfeldt was drained after seeing his team fall to a fourth defeat in their past five matches.
"We had a great chance to turn everything in our favour. Now we owe it to ourselves to give ourselves a shake and not make the same stupid mistakes as we did today," said Kohfeldt.
"I will not find a tactical solution in a week's time to make things different. It's not a matter of tactics … I'm just brutally empty.
"There was a great chance today – it isn't the case that it would have been impossible to win here.
"It hurts me for the club, for all the staff, for the fans. We'll try everything next week. Now I am sorry, I have to let it sink in first."