How to stay with a cheater?

Saturday night my fiancé/baby daddy went out with his two mates . He said he didn’t want a big night so they were just going to have drinks then go to the arcade.

2:30am I wake up to feed my two month old and my partner isn’t home,, I call him ten times and no answer, finally he answers and acts shady and keeps hanging up on me.

He gets home and calls me a psycho , I ask for his phone, he gives it to me and continues to call me psycho.

I go on it and find out that he was actually at a nightclub with a bunch of girls.

The next morning I asked if he had talked to any girls, he said no not at all. I said do you swear you didn’t talk to any girls? He said yes I swear!

So I said the names of the girls he was with and asked “so who is Steph, holly and Paige ?” He absolutely went silent and didn’t respond.

He then said “oh it was a quick hello so it doesn’t even count”.

I didn’t believe him so I messaged one of the girls and she told me he was buying them drinks, dancing with them, flirting with her, didn’t tell her he had a family and she kissed him but apparently he pushed away and said he has a partner.

After this message I go back to my fiancé , ask him again what happened last night? He said again it was nothing. So I said, this is your time to come clean because I’ve been messaging one of the girls and she’s told me everything.

Did you dance with her? He said yes. Did you buy her drinks? Yes

Then I went on to tell him how disgusted I was that he acted single. And then I said is there anything else? He said NO!

Me already knowing about the kiss , I kept having to say are you sure you don’t want to tell the truth because i already know! And he finally confessed about the kiss.

I’m so disappointed, I asked him why he did it and he said he can’t give me an answer, he says he was just being an idiot and was drunk and regrets it.

I can’t believe it, I wanted to marry this guy, we have babies together ! How do I get over this and forgive him and stay with him ?!

It’s moreso the fact that he lied to me continuously the next morning and was calling me the crazy one!

He’s now saying he effed up and he’s so sorry and he will make it up to me for the rest of his life