Found out my BF [35M] has been cheating on me [35F] the entire time we've been together (5 years) with his co-worker [34F] Need advice on how to confront him and move on.

My boyfriend and I have been together for five years. I found out recently that he's been cheating on me with his co-worker for pretty much the entirety of our relationship. How I found out: My boyfriend was driving and gave me his phone to look up directions to a restaurant. An iMessage popped up from a user identified with just two initials. I saw it, and brushed it off. But during the dinner my gut was telling me something was off. Later that night, while he was sleeping, I took his phone. Spent the next two hours reading three-years worth of text messages.
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TL;DR; He's always carried a torch for this woman and only started dating me because he was lonely and miserable without her. How do I confront him? I'm going to break up with him, but should I confront him first? If I'm ending this relationship, do I even need to confront him? I just want to move out, write him a letter and move on. I'm so heartbroken, hurt and devastated.


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