Actual possession, as stated by Foreigner, is very rare. What is more common, and actually relatively common at that, is entity attachment. I've personally had to release multiple entities in multiple sessions of healing facilitated by a trusted and experienced teacher of mine.
That being said, unless you (LandsUnknown) have strong reasons to believe you've taken on an external entity since you've looked in the mirror, you're most likely appropriating known spiritual phenomena to new experiences out of fear.
That being said, unless you (LandsUnknown) have strong reasons to believe you've taken on an external entity since you've looked in the mirror, you're most likely appropriating known spiritual phenomena to new experiences out of fear.