Lying partners

Well where do I start iv not done anything like this before but here goes last December I found out my partner was sleeping with another girl he meet at work the texts on the phone was very detailed any ways they both put me thought hell I was 34 weeks pregnant at the time and with everything that happened I became ill and was in hospital for about a week anyways me and my partner got bk together a few weeks after our little boy was born and then I find out he's was messaging here again people have sed to me that I'm stupid for getting bk with him he hides his phone never picks up when I ring and is always lying to me about where he's been in my head I think he's seeing her again but everytime I ask him he says "if ur gonna carry on bringing this up then I'm leaving you" I just Duno wat to do