The actress posted the pre-op clip Wednesday, dating it "January 16, 2023" ... rattling off questions while gearing up for her big surgery. Shannen writes, "I am clearly trying to be brave but I am petrified. The fear was overwhelming to me. Scared of all possible bad outcomes, worried about leaving my mom and how that would impact her."

Shannen admits she was worried she'd come out of surgery and not be the same person anymore -- ending with, "This is what cancer can look like."

For those unaware, the "Charmed" star was diagnosed with breast cancer back in 2015, but revealed earlier this month that she now has brain metastases ... a condition where cancer cells spread from their original spot to the brain.

She showed the medical team at Cedars-Sinai forming the mask she'd wear during radiation, saying she had her first round just days before her surgery.

Of course, this all took place in January -- and while she hasn't spoken about any updates, she told fans at a "Charmed" panel in March she's "feeling great."