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Villa have turned a corner in terms of form in recent weeks, and looked like they were well on their way to a victory, after Jhon Duran had headed them in front in the second half.
They were given a let off late in the game as Chris Wood's equaliser was chalked off for a marginal VAR offside call.
But they were punished just seconds later as Nikola Milenkovic headed in an equaliser. The drama wasn't done there though, as Villa tried and failed on numerous occasions to clear the ball, before Anthony Elanga fired in the winner deep
Emery insists his side lost control of the game in the final 20 minutes and it cost them - while insisting that Forest, who have moved into fourth and three points ahead of his side, after now one of their closest rivals.
He said: "It was under our control but in the last 20 minutes we lost control because they started pushing more and creating chances.
"We need to be consistent with the ball and try and be better.
"Of course it is a process, we are in a process and we have to continue to build the team, that is clear to me.
"It is a game where we needed to continue with our process.
"Forest are now a rival to us, they are playing very well and we were speaking about how good of a test it would be and they are a strong side.
"We had a plan for the 90 minutes but of course the opposition have players and moments, and they won because they deserved it in the last 20 minutes."
Emery also gave an update on defender Tyrone Mings, who was withdrawn from the starting line-up before the kick off at the City Ground.
He was suffered from an illness and was expected to be able to play, but pulled up due to sickness in the warm up.