Forrest struck just as it appeared the match was going into extra time after goals by Kris Commons and Georgios Samaras had brought the Scottish champions level.
It saw Celtic book their spot in today’s Champions League draw and Forrest said: “It’s the best feeling ever. We are in the Champions League, where we should be.
“We’ve done it in 90 minutes and that is what we wanted.”
Commons lashed in the first goal on the stroke of half-time and he acknowledged it was vital timing.
He said: “We got a massive boost. We knew with this sort of support we could go on and win the game.
“You can see what it means to the fans – it’s magic.”
Neil Lennon’s men beat Barca 2-1 at Celtic Park in the group stages last season on their way to the last 16 but Commons insists the win over the Kazakhstan champions was better.
He said: “I thought beating Barcelona was one of the highlights of my career. Certainly winning the double last year was one also.
“But this eclipses everything. To come from two goals down and to win it in the 90th minute was one of the greatest moments of my life.
“Going into the break with the goal was a massive twist of fate for us.
“It changed the complexion of the team talk. We thought, ‘we are half way there.’
“But as regards important goals, I think James will get all the plaudits for getting that goal in the last minute.”
Asked who he would like to face in the draw, Commons replied: “It’s just going to be another magical time seeing who we do get.”
Shakhter boss Viktor Kumykov had no complaints about the result.
He said: “We are disappointed but the result was fair. We lost to a team who were better than us, with a higher class of player and more quality.
“We lost a stoppage-time goal in the first half and in the second half, which is a pity, but now we have to get ready for the Europa League.”