How can I stop my girlfriend from cheating on me?

We were in a long distance relationship for 2 years and I recently moved back to Canada to finally be with her. Problem is there's some resentment from her because she feels I've never really been there. Last year, she was really sick but I had to move overseas for a job contract. I still feel bad I left her when she was sick, but she knows I had to do it for work. I am so grateful that she waited for me and remained faithful, which is why I know she is the one. We been living together for 6 months and she's been paying for most of the expenses because I am still looking for a job. She's always complaining that she's unappreciated and I feel bad for that. I want to give her the world but I need time to find a good job. Lately, she's been talking to a guy who's in love with her and wants her to leave me for him. She's turned him down but I can see how frustrated she is and I'm scared she'll leave me to be with him. What can I do? She's the love of my life and I hate that my current unemployement situation is ruining everything.