He got another girl pregnant UPDATED

Ok so I need some advice. This is a tough one. My boyfriend and I have been together for 9 years. We had a rough patch about a year ago and we took a break from one another. I moved to my moms with our daughter and he stayed home. We still talked almost every day and we still slept together. I asked him time and time again was there anyone else and he always always said no. He promised me there was never another woman. I never slept with or talked to anyone else. We got back together about 3 months after, moved back in and all. Well we had a camera installed on the front porch a while back and it broke off so I logged into the internet account to try to request a new piece and find out how to get it repaired. I saw there was some footage that recorded for whatever reason and that it was full. I was going to delete it to free space and saw then that he had another woman over at our house. She stayed over for a while according to the cameras timer. I immediately recognized the girl and searched her on Facebook just to be sure. She had me blocked. So I went to instagram. Saw that she had just had a baby. Also saw that a mutual friend had posted the babies pic with my boyfriends ethnicity. He is mixed and she is white. When I put two and two together and found out that was his newborn child I lost my fucking mind. He denied every bit of it at first saying that kid wasnt his and bla bla bla. I finally contacted the girl and she assured me it was his child. He then swore he wore a condom. What a fucking liar. I then found out I was pregnant. With twins. I am having the hardest time ever. He told me it was a one time thing and that he never even liked the girl (she is really bad looking and weighs like 300 pounds) and that he wanted nothing to do with the child. I moved back with my mom but I am still devastated. He is trying to be a family again and I just dont know how to deal with this. I keep having these "visions" of him sleeping with her and was it on my bed (they both said it wasn't) and why would he do that to me and just horrible nightmares. I love this man but he has destroyed me with this. WhT should I do? How should I cope? Should I stay? He sees nothing wrong with what he did. I mostly see he LIED about possibly the biggest thing possible! Someone please help me.

UPDATE; Thanks for all the feedback so far. I know I should've left a long time ago but it is very very hard when I have SO MUCH invested. He is very good at manipulating me and making me buy into his shit stories. He tells me that he missed me and was trying to fill the void, tells me that he was drunk, tells me tons of things anyway. And I have to be honest, I suppose I am fairly weak to him. He is the only man I've been with like this. I've slept with people before him but never had a serious relationship lasting longer than a few months. All I know is him. And it makes me question everything that I know now cause I feel like it was all lies. I am scared, I am alone and he is all I've ever had. Don't have much family or friends. He has been my main support for so many years in so many ways. He uses this in saying I shouldn't leave him for this and he didn't cheat and he didn't owe me the truth about something he did while we were "not together"

I guess I know the answer here, it's just so very hard to leave someone who has indefinitely been your other half for so long. How do I find the strength to get away, stay away, but still have contact for the sake of the kids??? I just can't imagine this and it pains my heart to imagine him having a family and doing the things we have done for so many years together with someone else.

If you have read this far, thank you from the bottom of my heart. I've never poured out my soul like this to people I don't even know.