Leigh Griffiths grabbed a double as the hosts twice led and then fought back with a late equaliser to share the spoils against their struggling visitors.
Although it was a goal-feast enjoyed by the 9,000 fans, Fenlon was not so forgiving and said: "It was a nightmare. Where do I start?
"The positives. I thought we played well and our two young kids out wide, Danny Handling and Alex Harris, were different class.
"We created a lot of chances and I thought we deserved to win. But when you defend like we did today, you are not going to win games."
At the other end though, Griffiths did receive his manager's compliments after he banged in his 27th of the season.
"The first goal he scored was as good a goal you will see," Fenlon added. "He's in with a shout of winning player of the year.
"He's a special striker. He scores all sorts of goals as we saw today. He's got a bit of a habit of scoring against St Mirren so they will be glad to see the back of him I suppose."