The Irish led 12-3 when Heaslip was yellow-carded in the 43rd minute for collapsing a driving maul as the Springboks attacked their line.
In his absence South Africa scored 10 points to establish a lead they retained throughout a dominant second half.
"We started well in the first half and built up a good lead," Heaslip said.
"We came out in the second half and as captain I should lead by example, but being yellow-carded wasn't a good example.
"I put my hand up straight away when we came in afterwards. I'm a better servant on the field than off it."
Four penalties from Jonathan Sexton gave Ireland a 12-3 lead they deserved, but they were picked apart after the interval as South Africa finally woke up.
"We were a little bit frustrated we weren't further ahead at half-time," Heaslip said.
"They were quite clever about how they played the game when they had the ball. I don't know how many opportunities we had.
"But we were still very confident and focused on what we had to do. We knew they'd have a purple patch, but felt we could keep our momentum."
The result condemned Ireland to their fifth successive Test defeat, their worst sequence since 1998, and follows their 60-0 drubbing by New Zealand in June.
Head coach Declan Kidney admitted they had made little progress on the pitch.
"The last game in the summer was unacceptable," Kidney said.
"This is a step forward in terms of bringing guys through, but we're here to win matches.
"That's what we get judged on and that's what we have to do. You don't pull on the jersey to come second.
"They got better field position and then started their mauling game.
"They smothered the ball for long periods of time and then we didn't get fast ball when we were in the positions we wanted to be in.
"We lost the game, but four new guys won caps and they'll gain from this experience.
"This is a tough learning curve and we need to get something out of the match against Argentina here in a fortnight's time."
Springboks coach Heyneke Meyer was delighted with the way his side were transformed after the interval.
"There were a few harsh words said at half-time. The first half was unacceptable, nothing worked," he said.
"The first half was the worst half I've seen, but the second half was the best we've played all season.
"We showed a lot of character coming back like that and I'm very proud of the team."
South Africa prop Tendai 'Beast' Mtawarira withdraw from the team this morning after suffering heart palpitations.
"He's much better and is out of hospital now. We'll make announcements as soon as possible," Meyer said.
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