help!! i’m freaking out!!

My dad owns his own company and he runs it out of the house and so all his workers meet at our house to go to work. i sometimes work for my dad. recently one of his workers and i got a little too friendly and we ended up having sex. i found out i’m pregnant. i’m freaking out, it was a huge mistake sleeping with him. Im 18 and he’s 26 and he has a girlfriend. what do i do? my parents don’t know that i’m having sex. someone help

i’m 100% sure it’s his. i haven’t had sex since june before him