Should I end my relationship?

A few months ago I caught my girlfriend of 5 years cheating on me with one of my former friends. This is the second time I caught her cheating on me with him, but her third time cheating. Because I'm so in love with her I forgive her for cheating. And we worked it out. But lately she's becoming distant again. She's been hanging out with one of her male co-workers a lot more. She hasn't touched me (sexually) since Valentine's Day. Part of me thinks she's cheating on me again. But part of me doesn't want to think that because I love her so much. I'm thinking about ended my relationship because even though I love her. I don't want to be cheated on for the thirds time. She knows how much I love her, and uses it to her advantage. How can I stop myself from being so dumb?
Should I end my relationship?
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