Jennie Garth is getting real about life after "Beverly Hills, 90210" ... and the money mistakes she still regrets.

On Dax Shepard’s "Armchair Expert," Jennie admitted she never really thought about finances back in the day -- cash was flowing in, she was spending, and she figured it would just keep coming ... until it didn’t.

When the show ended, reality hit hard -- Jennie says it was a serious adjustment once the paychecks stopped rolling in.

Watch the video ... she says it got worse -- she revealed her business manager basically scared her into selling the home and ranch she’d just bought for her kids ... a move she made, and now deeply regrets.

Lesson learned the tough way -- Jennie says once you fixate on a fear, you almost manifest it ... and in her case, it was the money drying up.