What to do?

Hey so me and my boyfriend been together 4 going on 5 years now we recently had a baby together who’s now 3 months old I recently found out he have had a 3rd phone since me being pregnant and how I found out was he was texting my cousin from it and lord knows how many other girls my cousin ended up telling me after the fact I wish she would have told me as soon as it happened but instead she kept coming around as if nothing happened remind y’all during my pregnancy she wasn’t around me like that but before pregnancy she was around so I feel like you haven’t been around in almost a year so she came to see the baby and after that day he started texting her I know deep down I can’t trust either one of them because I feel like she played along and he just not shit it’s so hard dealing with this because I know my family is over this shit unforgivable but how should I deal with her? When I confronted him he told me little things that she was doing like flirting and things like that and how can I just free my self from him we live together and just had a baby that’s the most hurtful part ? I’m not feeling nun of them no more but am I wrong she did tell me or did she just tell me to hurt me because why play along or keep coming in his face and not telling me I think both of them shitty especially him because he have a family to loose