How do I phrase a break up?

Lately (last few months or so) I have started to feel less into my boyfriend than I should be. He's abroad half the time because he is not happy here and when he's here he is constantly complaining about something or other. Which is really turning me off to the point where he told me to look at engagement rings and I can't be bothered and I'm semi obsessed with getting married.
Long story short, I need to let him go but I've never broke up with anyone before so have no idea how to phrase things in order to be clear but not bitchy?
How do I phrase a break up?
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