However, the former 'Dawson's Creek' actress and Jamie, 50, finally went public with their relationship in 2017, when they were pictured holding hands while walking along a beach.
And prior to their split, it was suggested that the couple were both becoming increasingly comfortabale with the interest in their romance.
A source said: "They're starting to be more comfortable about their relationship. They'd both been in public relationships before. They care about each other a lot and they tried to stay as quiet as possible because they didn't want to ruin that ... Jamie brings a lot of joy and happiness into her life. They've definitely become more serious."
Despite this, Katie - whose divorce from Tom was finalised in 2012 - continued to regard Suri as her first priority.
The insider added: "She's an amazing mom. She'll choose her family over him any day ... When they're together it works. When they're not, they're on their own. It's what works for them ... Katie found happiness with Katie first.
"She's worked her ass off since the divorce and she's finally gotten to a place where she's made a name for herself and can be her own person. She's back to her old self and it's great to watch."
