Richie Wellens described his Swindon side as not good enough for promotion following the 0-0 draw at Cambridge.
The goalless draw left Swindon four points outside the play-offs with only three games to go as the Robins missed the chance to move two points closer.
Cambridge are practically safe in League Two after moving nine points clear of the bottom two.
Wellens said: “Overall I’m not happy. For the position we’re in, I’m not happy at all.
“I’ve kept it in the back of my head but I feel this group haven’t got a promotion in them. Forget technically, forget ability on the pitch, I mean with character.
“I’ve been involved as a player with promotions and I can smell the atmosphere, I can smell the feeling between the players and I think we lack that, hence we’re falling a little bit short.
“It’s a shame because we were six games unbeaten. We’ve played some nice football; that’s probably the longest I’ve kept the lads in the changing room for a long time.
“In the last six or seven games, I think it’s the worst we’ve played. That game could have gone either way; we could have won it with a couple of opportunities at the end, the keeper’s pulled off some good saves, but you can’t just play at 50 or 60 per cent and think ‘I hope we go 1-0 up here’.
“The next three games are massively important for some of the squad because I’m not going to carry anyone next year, so they’ll be playing for contracts. I want to go out on a high, I want to win games.”
Swindon started strongly and Kaiyne Woolery blazed over after Kyle Bennett’s surging run inside 10 minutes. Woolery also struck a powerful effort from distance which was pushed away by Dimitar Mitov.
Cambridge grew into the game and Michael Doughty denied the hosts with a goal-line clearance from George Taft following a Jevani Brown corner 10 minutes before the break.
After the break, Harrison Dunk ran half the length of the pitch and teed up Rushian Hepburn-Murphy to fire wide, though Swindon almost went ahead 20 minutes from time when Mitov saved and held Doughty’s piledriver from long range.
Hayden Coulson’s cross was destined for Jabo Ibehre in front of goal, only for a defender to divert it behind for a corner to deny Cambridge the chance of a winner.
There was still time for Mitov to save from Doughty again, brilliantly tipping his free-kick wide late on.
Colin Calderwood lauded “a really strong point for us” as Cambridge responded to a heavy defeat against Newport last weekend.
He said: “Obviously if we’d snatched some sort of winner then we would have been safe but we’ve got another three games. We’ve got to make sure we apply ourselves, certainly with the endeavour and grit and determination that we showed today.
“We certainly nullified the threat that they posed in the early part of the game and I’ve got to give the team credit for that. I think the important thing was our defensive responsibilities today. We sacrificed lots of effort and energy just to try and cope with a team that started really well.
“In the moments that we did have the ball, we were a little bit disjointed. It got better from our point of view; it’s not wonderful but it feels wonderful, I must admit.
“It’s these moments that you can reinforce values and trust and importance of running, working for one another. We needed everyone to give their maximum today.”