We looked in control after goals from Dean Richards and Teddy Sheringham handed us a 2-0 lead at Loftus Road.
Milenko Acimovic had a goal disallowed but the home side somehow turned things around in the final half-hour as Junichi Inamoto, Steed Malbranque's penalty and Slyvain Legwinski secured a 3-2 victory.
Boss Glenn said: "After the first half performance we're scratching our heads as to how we played in the second half.
"The most alarming thing is that we lost the game from 2-0 up and it could have been five. That's how poor we were in the second half.
"It's a kick in the teeth. There are harsh lessons to be learned and we must learn them. There are no excuses, our second-half performance was just not good enough, with the ball or without.
"We needed to show more defensive resilience and quality. If we are going to achieve things, that's something which has to happen.
"Performances like that set you back and we have to bounce back on Sunday against West Ham."
Glenn Hoddle described last night's defeat at Fulham as a 'kick in the teeth' and insisted 'lessons need to be learned'.