Should I continue in this relationship

Me and my girlfriend have been together for 4 years, I had to recently move from Manchester to London for work which has been really tough on us but we were getting through it. 2 weeks before we went on holiday everything changed, she became very distant and didn't message or call as much, when i would go home at the weekends her normal loving attitude had changed. I became concerned i questioned her, she didn't know what was wrong and just said that she feels differently now. Late one night i couldn't sleep, i went to put an audiobook on her phone and found lots of messages from a guy from her work that she hadn't mentioned before. some of the messaged were inappropriate, he said things like how good she looked in those jeans and if she wants to kiss him or get a cake, she said cake but then followed up with weve only been to the shops 3 times, i'm a 5 date girl (sorry if this is too much info) this to me was too far, last night i questioned her on it. she seemed quite angry and didn't really apologise, she admitted what she done was wrong and i asked her to message him to say that she wouldn't speak to him anymore, then we would discuss it more this weekend. to this point she still shown me a message sent to him as i asked, she said she will do it in her own time as she is busy at work. I cant shake the feeling that I can't trust her anymore but really want to try and make things work. she promised me that nothing physical happened and i believe her, but how do we move forward from here? is it too late to come back? We've had a fantastic relationship for 4 years where we have had great trust and communication. I know she's upset with me for moving away but I dont think what she has done is justified. PLEASE HELP!