Sevilla had been snapping at the heels of pacesetters Real Madrid and Barcelona for much of the season but a run of three successive league matches without a win means that their promising title chase has unravelled.
Sunday's 3-1 defeat at Atletico Madrid left them trailing leaders Real by eight points and second-placed Barca by three. The top two teams also have a game in hand.
"It's difficult to diagnose what has happened but it has a lot to do with our emotional strength. We had great expectations in fighting for the title and in the Champions League and everything that didn't go our way killed our hope," Sampaoli, whose side were also knocked out of Europe by English champions Leicester City on Tuesday, told reporters.
"When you don't play well enough to win you feel frustrated and we have to return to our true selves and re-establish our identity.
"When things don't go how you expected them to, doubts appear and that happened today."
Less than three weeks ago, Sevilla were riding high after winning four successive La Liga matches which put them two points adrift of then leaders Barcelona and one behind Real.
They also had one foot in the quarter-finals of the Champions League after winning the first leg of their last-16 tie against Leicester 2-1.
However, since then they have completely lost their way, picking up just two points from three games and exiting Europe's elite competition following a surprise 2-0 second-leg defeat to resurgent Leicester.
Read the full article on eurosport.co.uk: Sampaoli blames Sevilla's freefall on lack of mental strength