2 year affair

I've just found out my husband has been having an affair for 2 years. We've been together 18 years and im only 34.
So were do i start im in hosptial giving birth to our son and i have a message saying my husband having an affair. I asked him and he told me everything.
Apparently his version is she came on to him they slept together 3 or 4 times he tried to end it and she threatened to tell me everything only way to stop her was to keep seeing her. I seen his messages he sent her as she showed me. All his had been deleted and removed call history.
Her messages was all him saying he loved her and wants to be with her. He tells me that he said this to stop her frim telling me. This women added me to fb a while agoas he works with her i thought nothing of it but he said that day she added me he was with her and tried to stop her from telling me as again he told her he didn't want to see her anymore.
The story goes on and on.
She told him 6 weeks ago she was pregnant and clearly could see on Facebook i was pregnant. He said she was lying about it and never was. He said she told him she lost the baby. She told me this was the second baby she lost. So not only did he cheat on me he never use anything for 2 years.
He admitted everything he told me it was a huge relief he could see no way out.
I was in hospital for 3 days given birth to our son i found out the whole time he was on the phone to her. She told him she was coming to the hopstial to twll mw everything he begged her not to and again told her he loved her and would leave me in a couple of weeks. Her friends then rang him saying he needs to go and see her now as shes taken an overdose he said no he's at the hospital with me. He then had her daughter her friends all threating to tell me. So as im in labour he outside on the phone to her. 1 hour after i give birth he's again on the phone to her telling her once again he loves her. He swears all this was to stop her telling me. So 4 hours after i give birth i have a message telling me.
I don't know what to feel im dumb just gave birth to our third child one of my happiest days of my life turned into my worst nightmare.
I don't think i was shocked as deep down i knew something wasn't right phone hidden coming home late from work. Just hurts more that it was the day i gave birth i find out.
Now im sat here numb and don't know what to do how will i cope mentality financially with a newborn. How to i tell my kids who adore him
More how do i face the world with my happy little family my newborn child and tell everyone im a mug and stayed with a man for 18 years who lied and cheated on me. I just don't know what to do. Truthfully if it was me and my older children who independent i would of kicked him out there and then. The fact i was sat there with a newborn baby hormones every were i just went numb.
She also told me he hit her and strangled her cos a week ago cos she said she was telling me about the baby and that's how she lost the baby. Which i thought was a lie because he might be a p#$/k but hes never raised his hands to me. I asked him and he admitted he hit her and told her not to tell me but swesrs blind she has never been pregnant.
More im writing more i know what i should do but im so scared i just feel like my world has eneded and can't enjoy my newborn child.
Makes it worse im 34 shes 50!! Am i that bad he would do this to me for a 50 year old hag and i mean she looks like something from Jeremy kyle show and im not kidding. Why would he do tbis what the hell am i going to do?????