Adlene Guedioura's stunning winner handed Forest the spoils, leaving Wolves struggling in 17th spot and without a win in eight games.
With three points from eight games, Solbakken said: "After conceding the second goal we looked like a team that had not won for a while.
"You could see that in the way that we passed the ball. Safe balls went wrong and many players made the wrong decision. That is often down to a lack of self-belief and that has to go before we play Millwall on Tuesday.
"We have not lacked it before but you could see it after we went 2-1 down. We have to work on that because if you are afraid of something you will never turn the corner.
"We need to believe in what we are doing and I think it was a bit to wild west sometimes. I was not happy with the way we defended.
"It was too open and we allowed them to counter-attack. There are very small margins but if you have not won in eight games then it is also a bit in the head as well.
"When they scored the second goal we struggled to get the rhythm in the game and they made it difficult for us."