After one of the most difficult weeks of her life took a sharp turn for the worse, Jennifer Garner is proving her inner fortitude, and finding support in a tight network of family and friends.
"She's not a helpless deer caught in the headlights," an insider tells PEOPLE in this week's cover story. "She's much stronger than you know."
Any lingering doubts regarding her divorce evaporated when the 43-year-old actress learned of an alleged affair between her husband, Ben Affleck, and the family nanny, Christine Ouzounian.
Affleck, 42, vehemently denies any infidelity, but for Garner the alleged affair "was the final straw" and the "ultimate betrayal" after a difficult 10-year marriage.
"She's grieving," says a source close to her, "but she has an amazing support system. She just had no idea what was going on."
The West Virginia-bred Garner is leaning on her friends and family, including her mom, Patricia, and her sisters Susannah and Melissa. "She is very traditional," says a longtime friend. "That's who she is and that's what she represents. She's grounded and she's a fighter, but this is not what she saw for herself. She's shocked."
Another source close to the actress adds, "This is not anything she wanted as an outcome for her family or her marriage. But everyone's rallying around her."
Since returning from a family vacation from the Bahamas, Garner has been living with her three children, Violet, 9, Seraphina, 6, and Samuel, 3, in Atlanta while she films Miracles from Heaven. Affleck has visited the family often.
VIDEO: Inside Jennifer Garner's Heartbreak
At work, Garner has continued to live up to her reputation as the consummate professional. "It's very obvious that she is having a terrible week, but it hasn't affected filming," a set source told PEOPLE. "She made it clear that she doesn't want any special treatment. No one is sweeter."
When she's not on set, the actress has kept busy with family outings, including a recent mani-pedi day with her daughter Seraphina, and a trip to the farmers' market. "The kids and Jen both looked like they were very happy," an onlooker noticed. "They clearly adore their mom."
Moving forward, Garner remains committed to communicating with Affleck regarding the kids, and expects divorce mediation to be civil and swift.
For much more on Affleck and Garner's difficult split, the nanny allegations and how Garner is coping, pick up this week's issue of PEOPLE, on newsstands now