Emotionally cheating with ex

This is going to be long but I'll give the shortened version.

Im not married but have been living with my g/f for 4 years, were in our mid 50's so were by no means spring chickens.

I met my g/f within weeks of her getting a divorce but because they couldnt sell their house her and her ex continued to live together. Our problems started about 3 years ago when I found out what she was doing on FB. She was flat out flirting with other guys, telling them how wonderful they were and treating them better than she was treating me. She was asking these "old friends" for their phone numbers abut never told me what she was doing. When I found out about FB I gave her an ultimatum FB or me!

She chose me and gave me her word she'd never get on FB but during that time I knew something wasn't right so I checked out our cell phone bill and apparently she was spending allot of time texting and calling her ex and never said a thing to me about it. She lied one day and said she was going to go the gym and ended up talking to her ex for 90 minutes instead.

A couple of months later I saw she had sent him a text by than I had enough so I told her to send him a text telling him their friendship was over and its time to move on. When I asked her to see the text she said she deleted it, when I looked at our phone records one was never sent. She said she didnt want to hurt him again and wanted to do it by phone while she was at work the following day. I made her send the text right than and there.

I just recently found out that her and her ex have been going behind my back texting and calling each other for at least 3 years and she was the one that would ALWAYS call him. All I ever asked this woman to do is to be open and honest with me and her response would be "Im not doing anything I cant do in front of you" or "Im being completely honest" but shes been anything but.

When I found out how long they were in contact I was extremely hurt. For 3 years I begged her to be honest with me and she said she was. Now Im mad as hell because she intentionally deceived me and did everything possible to hide her emotional affair with her ex and intentionally hurt me in the process. Now that the damage is done and cats out of the bag she says she's sorry and wants to work it out but how can I ever think she's going to be honest with me when she hasnt been able to when thats all i was asking her to do?

Her and her ex have no reason to stay in contact, if they had kids together or had business related matters I would completely understand, her excuse for constantly contacting him without telling me is she wanted to try to ease his pain. Are you kidding me?