Alan Archibald's side were denied a win at the death as James Collins cancelled out a superb solo effort from Kris Doolan with just seconds to go.
Forbes spurned a glorious chance to put the Jags' 2-0 up with 15 minutes remaining in the Capital as he could only fire wide from inside the area.
And the midfielder knows he should have done better and all but clinched the points as Terry Butcher's side snatched a draw.
He said: "I have put my hands up to protect myself because I thought the ball was going to hit me in the face. It did hit my hand.
"It was a chance, though. I have hit it wide and I probably should have done better.
"I didn't connect well with the ball, there is nothing I can do about it. I was a bit gutted I didn't score.
"It was a chance I would love in every game and back myself to take it. It is just one of those things."
But Forbes - who has only scored once this season - admitted that Thistle might have been happy with a draw before the game started.
He said: "The way we have been playing and with the results in the last few weeks, we would maybe have settled for a point.
"But when you are 1-0 up in the 92nd, 93rd minute, it is hard to take.
"Every one of the boys was brilliant. It is probably a good point coming to Easter Road but we were still unlucky not to get all three.
"We played some good football, but they did have a lot of pressure on us, especially in the second half.
"They played long balls but we defended brilliantly."