Chances of wrong ultrasound?

Recently told I’m having a girl, but I’m so scared of being excited and then being told it’s a boy after getting everything ready. Was told at 20 week anatomy scan, she showed us the screen and pointed out the three little lines and said “yep, I’m pretty sure it’s a girl”. Why am I still so nervous?? She said she caught the gender earlier in the ultrasound as well. I just want to be excited but I’m so damn nervous that it’s wrong!