Scotland followed their 51-22 defeat by New Zealand with a 21-10 loss at Murrayfield, where they failed to get going as an attacking force until Henry Pyrgos crossed for their only try in the 51st minute.
Lamont felt their tackling was too high and not strong enough as South Africa dominated territory in the first 50 minutes, when all of their points were scored.
Andy Robinson's men face another physical challenge at Pittodrie on Saturday and Lamont knows they must be stronger.
"Frustration is the main thing," said the Glasgow player. "We are doing well in certain areas but we are just not getting the balance in defence.
"We are getting in people in the right place to make the tackles but we are just not hitting them hard, dropping them down and knocking them over.
"South Africa got the momentum going forward and they are very good at that and are going to punish you for that.
"The physical side of things is something that should never have let us down, but unfortunately it has been the same the last two weeks.
"We should have been confident because the summer tour went so well.
"They are number one and two in the world for a reason but the scorelines should not have been quite so big. We should have been pushing them closer and working them harder."
Scotland pushed the Springboks back for the vast majority of the last half hour but could not convert a series of chances thanks to some desperate defending, some crucial refereeing decisions and some poor decision-making.
And Lamont believes they need to be sharper and more physical going forward too.
"There are a lot of positives, our attack is going well in places," he said.
"But when we are going well at the breakdown they put a lot of pressure and made us have a lot of slow ball when we are attacking.
"We should have been hitting those rucks so hard and blowing them out so we can get quick ball. They are good at making a mess, they are big physical guys.
"That's where we have to be sharper.
"There are plenty of positives, we are scoring tries now but we need to get that balance spot-on of having a rock-solid defence and an attack that can cut.
"We are just not there yet."
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