Hearts, run ragged 7-0 when the teams met in the Scottish Cup three weeks ago, defended resolutely, taking their lead from an inspired display from goalkeeper Jamie MacDonald.
But Commons finally burst the Hearts defensive dyke with a clipped finish just after the hour, and Forrest settled it late on with a composed strike.
MacDonald set the tone as he reacted brilliantly to divert Anthony Stokes' early point-blank shot onto the bar before turning Commons' low drive round the post.
Teemu Pukki was the next to try his luck but MacDonald had his angles right at his near post as Hearts were happy to soak up the pressure.
Callum Paterson threatened briefly on the break before Joe Ledley - marking his 100th appearances for the home side - blazed over from 25 yards.
The Welsh midfielder came much closer as his lunging header from five yards flew to safety off MacDonald's knee.
But play became increasingly scrappy, forcing Neil Lennon to introduce Forrest as Commons found the side-netting with a 20-yard free-kick.
Virgil van Dijk headed over and MacDonald parried Commons' angled drive before the breakthrough finally arrived, Commons (64) slotting in his 12th of the season after Scott Brown's threaded pass.
The chances kept coming and MacDonald continued to frustrate - the Hearts keeper somehow deflecting Ledley's near post flick onto the frame of the goal.
Commons' header was tipped over, but the Celtic scorer had to head off the line from Dylan McCowan - Hearts first attempt on target - in the 90th minute before the lively Forrest skipped free and clipped home a cool finish in added time.