Why did my fiancé cheat on me just after we have a baby together ?

He felt left out but why not talk to me ? Ego boost ? It meant nothin it was just sex , it was more than one woman , it was a few one nighters but then started seein one of them , they were all slutty an trashy lookin with no self respect , the one he was seein believed that they was so in love an they had all the future planned they had known each other a few weeks , she told me a lot of thins that didn't add up then lied About bein pregnant by him as soon as she knew about me , she said he spoke about some one an said it was his ex wife but every thin he told her was about me , all gd stuff 'He had an ex wife' we have been together over 4 years an only last year he asked me to marry him , don't get me wrong I have suffered bad with anxiety before I was pregnant an durin an he always said that he missed the old me an wanted me back