Liv said she found out on New Year's Eve she tested positive for the virus ... "Suddenly on The morn of the last day of 2020 ... boom it took me down. It comes on fast, like a locomotive, Owchie." She goes on ... "With it Feelings of fear, shame and guilt swirling through you, who could you have gotten it from and who could you have infected ... Terrifying. Luckily the rest of my family and bubble were negative."

And, then there are the symptoms ... "It F's with your body and mind equally. Everyday different. Being isolated in a room alone for 10 days is trippy to say the least."

The 'Lord of the Rings' star, who has 3 kids, says, "I missed My babies beyond but they visited my window and called up to me and I watch them play outside. Such a gift. They sent little messages and drawings under my door. Reminders of what's on the other side. What to get better for."

What's interesting -- and scary -- Liv took all sorts of precautions. In fact, she bowed out of returning to film "9-1-1: Lone Star" last year because she was so concerned about COVID.

Liv has now been reunited with 4-year-old Lula Rose, 5-year-old Sailor Gene, and 16-year-old Milo William, posting a pic of her embracing the 2 youngins, with the caption, "Reunited with my loves ... what a wild 2 weeks."