The visitors had taken the lead against the run of play just after the half-hour in Saturday's League One clash at St Mary's, when Gary MacKenzie converted from a Peter Leven corner.
They doubled their advantage seven minutes into the second half when the impressive Sam Baldock latched onto a Stephen Gleeson pass before firing a powerful left-footed shot low past Kelvin Davis from 15 yards.
Saints looked out of the contest but two goals inside a minute from substitute Forte turned the game on its head.
His first, after 66 minutes, saw the former Scunthorpe winger expertly control Rickie Lambert's left-wing pull-back before volleying home from six yards. And 60 seconds later, Dan Harding was the provider as his left-wing ball was palmed into Forte's path by Dons keeper David Martin, allowing him to poke home unmarked from point-blank range.
The fightback was completed 13 minutes from time when Lee Barnard collected Adam Lallana's prodded through-ball to smash home from 15 yards.