i dont know how to move on.. im depressed.

I'm 18 weeks with my significant others child and well needless to say we have been fighting for the past couple weeks because I saw that he called an ex on Facebook. he said it was nothing and it was an accident. fast forward two weeks later and I'm still thinking about it so I look at his phone and there is a missed call on his phone log from her so I thought wtf he just got a new number couple months ago only family has his number so I looked back at his Facebook messenger call log and figured out how to see how long he was on the phone and turns out they talked for around 15 mintues and she called him back 10 minutes after the call and they talked for another 5. I brought it to my s.o's attention and told him I was done and he left for the day when he came home we tried to work it out cause we both love each other and our having a child together but then I lost my phone and went to grab his to call mine to see where it was and he put a passcode on his phone... I'm at a loss here . I'm so depressed and don't know what to do he says he is going to show me he loves me and wants to be with my forever and he did deleted the girl and blocked her but seriously a passcode now although he finally told me the code after we were done fighting I just can't help but think of my ex and him cheating and hurting me so bad.. everything just isn't adding up.. I need to move on from all this or it's going to ruin us but I can't .. I need help.