Following his Italian Grand Prix ban, Grosjean attracted fresh criticism when he crashed into Mark Webber's Red Bull on lap one in Japan.
The Frenchman then drove a subdued race to seventh in Korea.
Boullier believes the biggest issue now is that Grosjean has to stop being hard on himself and get back to driving naturally.
"It's definitely time for us to draw the line and move on, but more importantly it's also time for Romain to put it in the past for himself," said the Lotus chief.
"Despite a lot of pressure, he drove a good clean race in Korea and I fully believe he now understands what he needs to do to avoid these things in the future.
"Romain's target for the rest of the season is clear; to build on the platform he created in the last race and help the team push for maximum points in the last four races."
Grosjean acknowledged that he had felt under huge pressure in Korea.
"For sure I was a bit nervous starting the race as there was quite a lot of pressure," he said. "Myself and the team did a lot of work trying to understand a few things and it's certainly helped."
He agreed with Boullier that he now has to draw a line under his mistakes.
"I've been my own worst enemy at times this season, but hopefully now we can put it behind us and move forwards," Grosjean said.