Ryan Hall scored twice and Barry Corr and Kane Ferdinand were also on target for the Shrimpers.
Jackson said: "I'm bitterly disappointed with the result and it makes Tuesday night's game against Burton a must-win one now.
"Hopefully results will go our way but we can't rely on other results, we must get results ourselves.
"Having seven players missing through injury doesn't help and we were playing other out of position, but we should have done better than we have.
"We switched off and have gifted them three goals. We can't buy a goal at the moment, so it's important that we defend well and we haven't."
Source: PA
Source: PA