How can you be sure she's telling the truth after she cheated??

My girlfriend/fiance (24) and I (30) have been together for 3.5 years and now have 2 children together. We both came out of unfaithful marriages and swore we couldn't do that to each other knowing how hurtful that feels. Well year 1 was perfect and we were always happy and has a son together. Year 2 started great but quickly got rough due to loosing my job and we decided to move back to my hometown where we had family and I could find work. Well she also got a job working at a big car factory so she could help with bills. After awhile she started being a bit closed off to me and blamed it on being tired. When I would ask about her day she'd tell me about the people she worked with but I started noticing after awhile that she talked more and more about this one man than anyone else. When I told her it was making uncomfortable she just said it's because he's the one that was usually there to help her and nothing more than a friend... well long story short her schedule and drive time started being really sporadic and we argued more and at one point she decided to go stay with a girlfriend for 2 days so we could calm down. About a month later when we started doing better and was getting close again she came home from work one night and started crying and finally confessed that he wasn't just a friend and for a good while during our lowest time they would get off at the same time I so on her way home she'd stop by his house to just talk but according to her one thing led to another and he kissed her on the neck and in the heat of the moment she kissed him on the lips which instantly turned into making out and touching each other and led to them going to his bedroom where they got each other naked, she went down on him, he went down on her, and they ended up having passionate sex... afterwards she got dressed and came home and just told me there were wrecks and traffic had been stopped...this apparently became a regular thing for them for a few weeks and when she supposedly stayed with a girlfriend she was really with him, sharing his bed and both nights they had uninhibited sex. Well late the second day she started feeling guilty and ended up coming home saying that her friend told her we need to try to work our issues out. It took her a month to come clean about the whole to me and after a couple days of being devistated and asking her countless questions I agreed to try to work it out and get passed it, she quit her job and we ended up moving 3hrs away and started over. It's been a year and a half and I still can't get completely over it and now I don't know if I should trust her again but if I don't get passed it soon I fear it cause whatever we have now to end and even after everything I still love her to the moon and back