Alan Archibald's side threw away three points at home to Hamilton Accies as the ghosts of last season came back to haunt them on just their third outing of the new campaign.
The Jags looked to be heading for narrow victory when Kallum Higginbotham's spectacular effort broke the deadlock just five minutes from time.
But Thistle couldn't hold on as Tony Andreu levelled just seconds later before Jason Scotland netted in injury-time to leave Archibald's side with nothing to show for their efforts.
A similar problem plagued Partick all last season as a number of points slipped through their grasps and summer signing Stevenson couldn't believe history repeated itself on Saturday.
He said: "It's disappointing. We never played particularly well compared to the previous two performances which came after a good pre-season. We didn't do well enough yet found ourselves a goal ahead.
"I can't explain what happened after that - I've never been involved in a game where we've been 1-0 up with three minutes to go and lost. It's inexcusable. It's a lack of discipline, a lack of knowledge to win the game.
"I'm lost for words when I try to think of how you can lose a game from being in that position.
"It was naivety, but we need to try and learn from that lesson and make sure if we're in that position again the same thing doesn't happen."
Thistle now have one win, one draw and one defeat from their opening three Premiership fixtures this term, but it could, and should, have been far better for Archibald's side.
A FAILURE to see games out when on top cost Thistle seventh place, and maybe even a top-six finish, on their return to the Premiership last term.
The Jags have ambitions of breaking into the top half of the table this time around, but Stevenson knows they have to quickly learn their lessons.
He said: "Last season the boys struggled with games where they were in good positions and lost. We wanted to try and put that right this season, but it was the same problems on Saturday.
"I know it's only three games into the season for us but it's a major disappointment for us. If we want to achieve anything this season then we need to learn from what happened.
"It's a massive three points lost and we're just lucky that we've got a game so soon with Morton on Tuesday to try and get out of our system.
"It will be difficult going to Cappielow and cup games away from home are always difficult."
With their first three league fixtures played and four points collected, Thistle will turn their attentions to the League Cup tie with Morton tomorrow night as they look to get back to winning ways.
The Jags' have struggled in Scotland's top knockout competitions in recent years and former Hearts star Stevenson is determined to put that right as Archibald's side prepare for their Ton test.
He said: "There's no reason why we can't go on a cup run. Speaking to a few of the lads, they haven't been involved in many in the last three or four years.
"I've been lucky to get to a final and it's an amazing thing and a great day so there's no reason it can't be Partick Thistle."