The visitors looked set to hang on for victory after left-back Ben Coker had given them the lead with a 23rd-minute free-kick.
But after Coker was sent off in the 88th minute, Forrester pounced in the first minute of stoppage time to spare Bury from a fourth straight defeat.
The Shakers had started strongly, with Tom Soares having a shot turned wide by Southend keeper Dan Bentley.
The visitors replied with centre-half Mark Phillips' header well saved by Trevor Carson, but the Bury keeper was beaten shortly after.
Southend won a free-kick just on the right side of the box and Coker curled it in from a tight angle.
Winger Anthony Straker then caught Nathan Cameron in possession and squared for Freddy Eastwood, but his shot was smothered by Carson.
Southend's other winger Kevan Hurst volleyed narrowly over just after the break while Eastwood blazed another chance over.
As Bury struggled to turn their possession into chances, Will Atkinson fired wide on the break for the Shrimpers.
But Coker was dismissed after being booked twice in eight minutes and although Forrester headed home from substitute Jordan Sinnott's cross, Southend still managed to end their four-match losing streak.