What are we talking about here - you sleeping with him from time to time?
You also know about the girlfriend. And so I guess you're also a bit implicated in what's going on. If it were me I'd:
Either stop sleeping with him and tell him he's got a gf so can't have me too (sounds obvious, I know, but he is a bloke 😅)
Or (if you like having him as your booty call - after all, he is your children's father) tell him you can't do this with him also having a gf, so he needs to ditch the gf. No gfs allowed.
To be honest, it really sounds like option one is the most sensible. Stop seeing him, leave him with his gf(s), find someone else.
But I'm afraid that although you say you don't love him, it doesn't sound like that.
Oh, and the one thing I wouldn't do would be to tell the current gf. None of her business. This is between you and him. And you seem still to be the no. 1 for him. So maybe just decide honestly what you want, and tell him.
It sounds like he wants you. And I suspect you want him too. But not to live together again. So big decision time.