This is going to sound COMPLETELY off the wall and random, but that's just who I am.
Yesterday after my situation I came up with a good analogy: Relationships are like a Bar of Soap.
You know how you keep using the bar of soap even though there's basically NOTHING left on it and you don't want to open a new one until you get everything out of that bar of soap? But then you get all happy and excited once you open that new bar of soap because of how much bigger/easier it makes lathering up everything

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Basically a relationship is like that bar of soap. People keep using the same relationship trying to keep it for as long as possible even though there's nothing left, but they're just too afraid/lazy to grab a new bar of soap (new relationship). But when they finally do come around they wonder why they didn't just toss the old soap (relationship) away for the new one (new girl).
Hopefully this helps/makes sense. It's helping me with my current problem

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