relationship with kids

I’m having a hard time finding how i’m gonna balance my new partner and my kids😩 I have two girls and am so scared to introduce someone who isn’t their dad.. how did you ladies do it? tips? I will gladly be open to any opinions as well.. i’ve known this guy for a few years now and he has seen my first daughter but we are communicating and talking more he’s been coming over to the house but I haven’t really been letting my kids be interactive with him just yet. and their dad is gonna be hell bc he doesn’t want anyone around them.. which is nuts bc someone else eventually will.. idk i’m at a lost