The actor's most recent ex-girlfriend, Saturday Night Live producer Lindsay Shookus, also tried to get him help after she watched his "normal behavior" change several weeks ago, a source tells ET.
"Ben had been drinking and Lindsay was encouraging him to stop," the source claims. "At that point, he still didn't seem to feel he had a real problem. Sadly, he spiraled very quickly, and Lindsay felt helpless."
"Lindsay wanted Ben to go to rehab, but had trouble getting him there," the source adds. "He didn't want to be controlled and ended things with Lindsay."
Garner, who drove Affleck to the facility earlier this week, now feels "relieved" that the father of her three children -- Violet, 12, Seraphina, 9, and Samuel, 6 -- is "in a safe place," according to our source.
"This has been a rough few days and [Jennifer] wants nothing more than to feel [Ben] is getting the help that he needs," a source said. "She is used to putting on a brave face, and handling Ben's addiction, but the process has been very exhausting for her. She won't give up on Ben because he is the father of her kids."
Hear more on how Garner has supported Affleck over the years in the video below.