Cheating

my fiance & i have 2 kids together & have been together for 3 years. over a 2 year period i have caught him talking to other females 3 times, once while i was in the hospital after having our second child. recently he had come home around 3 am & he had brought over one of his female friends (which is fine, im not going to tell him who he can & can't be friends with) but he explained that she had come over to spend the night on the couch after losing a close friend. about an hour later i come out the the living room to make my son another bottle & she has a pillow in his lap laying on him & he has his arm around her. I automatically think something is going on because of what has happened in the past & go back to bed without another word to him. i wake up the next morning & hes not in bed with me because he stayed up with her all night. after she finally left i talked to him & he started getting super defensive & getting mad at me for accusing him. finally we get over it & the next day I'm on his phone & open a message between them both & hes asking to go over to her house and cuddle with her (she shut him down thankfully) & he said he only did it because i was accusing him so he decided to go cheat since i accused him of it and supposedly he did nothing wrong. i let it go to stop fighting with him but how do i get the constant feeling of him cheating out of my head? how do i trust him again? we have 2 kids and i really dont want them to grow up without their dad in their lives..