They've been keeping a secret.

Ben Affleck's girlfriend Lindsay Shookus left the actor's Pacific Palisades bachelor pad this morning and headed to LAX to catch a flight to NYC, and she seems slightly overwhelmed by all the attention she's getting. While we're sure she had to get back to work producing SNL, she also might have skipped town to avoid the paparazzi.

The Oscar winner and Shookus spent 4th of July together, after the A-lister put in some time with his kids and ex Jen Garner at a neighborhood parade. Last week, they traveled to London together, and last night they were spotted on a romantic dinner date at Giorgio Baldi. It might seem like a whirlwind new romance, but it's not...

Sources tell us that the two have been seeing each other on-and-off for years, and that Shookus is the real reason behind Ben and Jen's divorce. The Boston hunk and the blonde first met before he was ever with Jen, then cooled off before picking up where they left off when the Hollywood power couple began having problems.

Our source tells X17:
    "Jen never wanted to divorce. When she found out about Lindsay, she confronted her during a trip to NYC. She told her she needed to back away because they were working on fixing their marriage, and Lindsay wasn't having it. She basically told Jen that she's been with Ben longer, and that they're in love."

Another source tells us Jen tried to get Ben to end things with Lindsay for the sake of their kids and keeping the family together, but he simply couldn't.

"Ben has felt for a long time that Lindsay is his soulmate. He realized a couple years into his marriage with Jen that he had settled down with the wrong woman, so being with Lindsay has always been his plan. He didn't want to hurt Jen, so it took him years to get out of that relationship. He couldn't be happier to finally be able to go public with Lindsay."

We seriously can't imagine how Jen is feeling right now.