How do you know if she`s the one?

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I have had a couple of long term relationships ( 7 & 10 yrs) and in both cases thought "this is it, this is the woman I`m gonna be with the rest of my life". Both times I have been wrong. Last year my long term girlfriend cheated on me and left me for a guy who is 12 yrs younger than I am (we were both 33). I started seeing a girl in summer last year and things are going well. At our age we have both been hurt and have taken things slowly. We have a holiday booked for early next year. Im starting to get that feeling of "This is the one" again. Am I clinging onto false hope? Could she be "The One"? How do you know when you have found the right partner?