The Shrimpers would have moved up to third with a victory, but two sloppy first-half errors were punished by the visitors and they instead find themselves down in ninth.
Alex Woodyard needlessly brought down Russ Penn inside the penalty area as Marlon Pack gave the Robins the lead from the spot, before a defensive mix-up between Mickey Spillane and Paul Smith led to Paul Benson adding the second.
Sturrock's side improved in the second period but only had Ben Reeves' late consolation to show for their efforts.
"We gave away two ridiculous goals and the second one was just farcical," said Sturrock.
"The second half was much improved.
"I thought we dominated it but it was too late after the goals we gave away and I also hold my hands up to not picking the right team from the kick-off.
"The injuries aren't helping and it's unsettling the team but we can't use that as an excuse.
"We have another big game this weekend against Northampton Town and we have to make sure we start better in that than we did."