The Dons have now not won in eight games and Glass blamed individual errors for costing them the win in Paisley.
Aberdeen were 2-1 up through goals from Scott Brown and Christian Ramirez when Teddy Jenks was sent off early in the second half.
Connor Ronan scored his second of the game and Marcus Fraser netted the winner to see Jim Goodwin's men over the line.
Glass said: "If you are a footballer, it comes down to your individual performance. Are you capable of staying in the team and affecting the team positively?
"We've got a big group so the players are going to know there is competition for places right across the board.
"It's something that's real at a club like this. You cannot continue making mistakes and expect to stay in the team.
"I thought we did OK up to the red card which massively impacted the game. We lost a couple of poor goals after that and that's it in a nutshell.
"The disappointment was in the goals we gave away. A good football team like St Mirren pick holes in you. It was important at 2-1 to make yourself hard to score against and we gave them goals.
"It was a good finish to make it 2-2 but if you give teams goals you're in danger of losing games.
"Any time you go down to 10 men it will be difficult and if you lose goals like that it will be difficult. But there's no hiding the fact that for 50 minutes we looked like we were going to win."
Goodwin was delighted to finally get a cinch Premiership victory at the seventh time of asking.
He said: "We had to work hard for that win and now we have to try to kick on and build some momentum.
"We're not going to get carried away as the red card was a huge turning point in the game. Going in front it was frustrating to find ourselves 2-1 down but we now have to try to build on this win.
"The lads are still getting used to the way we want to play. But I couldn't be any happier just to get that first win and get the monkey off our backs."