Unsure what to do!!

Never done this before, but here goes.. - sorry it's such a long post

I have been living with my fiance for 2.5 years, and We have gone through a huge amount together to get where we are. Things were going really well, But about a year ago she met someone at work who she talked and text to secretly on the phone. A few months after she finished for the company, I looked at her phone one day and she had text this him "I love you too".

I sat her down and talked to her about it, she was very mad at me and nearly walked out, as I broke her privacy (which is fair enough). She admitted that she thought she loved him but after she finished at the company the spark left.

We got through that and things were again going well, she then started at another company which she is enjoying alot as she has a great work social life.. She has started going out with them every friday night, and comes back the following morning... this happens every friday!!

I swore to myself I would not look at her phone again but I did... This time I saw that she had taken pictures of herself (in her bra and knickers in various poses, in and out of bed) and sent them to some men from work saying "Just image what I can do in bed", and "Thought this would cheer you up"..

I don't know what to do, I know if I confront her again she will walk out.. I still love her, but the knowledge of those messages is eating away at me.. and I can't be sure that she is cheating on me..

What can I do??

Please help