Mass growing in uterus/surgery Friday

Anyone have this before and it turn out ok? They said it could be placenta left over from when I had my daughter 8 months ago, fibroid, a fetus that never grew, or rare but cancer. I am freaking out and need some help or reassurance it can be one of those other things. I thought if placenta was left inside you, you would go septic and die if they didn’t get it out right away...? So 8 months later how could it still be there without making me sick etc? Anybody have something similar happen to them? Help please