How do you break-up with a girlfriend who appears seriously entertaining a man w/c by chance I found out?

My girlfriend, who I sleep with and is practically my wife as I support her extensively, was acting strangely when we went out of town and I found out she was calling someone who was courting her and who I believe has had intimate words with him and they have been exchanging "I Love You" to each other altho they are just starting and have not even dated yet but have met once. The guy works outside the country. This is at least the 2nd time I found out. I am hurt of course but I love her so much. But now I want to end the relationship because I feel betrayed. I told her why didn't she just tell me about it and I can accept her decision and leave. She was confused because the trouble is she needs me for economic reasons and will be displaced. I don't want this to happen to her because I love her but I don't trust her anymore. How do I break up with her?