Help me please ladies...

What do I do when I just can’t seem to find trust for my partner?! I want to so bad but he’s hurt me before and I came back home literally the next day after I found out he cheated on me with his ex wife( like literally kicked me and our 5 month old baby out) and then moved in with his ex wife into her house! And was still sleeping with me and seeing me 3x a week. He did that for 3 months and also gave me an infection from sleeping with her! He says he swears she was the only one but I don’t believe him for some reason.... we’ve been back together for about a year and a half now and have another baby together.. we’ve been together 4 years all together. I just can’t let go and I can’t trust him at the same time...