The Royals made the brighter start and deservedly went ahead thanks to a comical own goal by Villa defender Nathan Baker.
However, the visitors hit back through goals from frontmen Christian Benteke and Gabby Agbonlahor to leave the Berkshire side in serious trouble with nine games to go.
McDermott told Sky Sports: "It's so disappointing. We took the lead and I thought we were good value for it. I thought we were going to kick on, but we shot ourselves in the foot.
"We've given two goals away just before half-time and that's something we need to learn from. It was a real body blow.
"We have to take this defeat on the chin and we must learn that when you get yourself into a lead in this division, you have to capitalise on that but we certainly didn't do that today."
With away matches at Manchester United and Arsenal to come for Reading, McDermott's believes his players should relish the opportunity of taking on the big boys and insists his side still have a chance of survival.
"We still think we can (get out of trouble). QPR have won two games on the trot and I bet if you asked (manager) Harry Redknapp if he feels he can get out of this, he would certainly say he could."