I don't know what to do .

Please no rude comments either . 
I just found out today that my fiancé has been going behind my back and hanging out with a coworker ( young pretty girl that's his age) . He texts her and goes behind my back and deletes her texts. I found out my using his MacBook that he was doing that . He's apparently been messageing her everyday non stop for 4 months now . And I'm 8 months pregnant begging to get his attention when he is not sleeping . I live with him and I see him one or two days a week. That's it. he works 12 hour shifts at the hospital and when he's not at the hospital he's going to college . This girl works with him and goes to school with him . They've been studying together and apparently during the whole hurricane mess they stayed together in the same room at work , alone . He did not once mention to me about a Megan but he talks about his other coworkers like it is no big deal. The fact that he has hidden all of this from me has hurt me terribly . I feel like I'm not good enough for him. He doesn't see it as cheating at all. He find nothing wrong with it . I just don't know what to do . 😔