50 down and 50 to go...but where has my motivation gone?

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I lost 52 pounds in 5 months almost effortlessly, and I have at least 50 more pounds to lose, but my motivation has gone on hiatus and the weight loss is not as effortless as it was. I turn 40 this summer and would really like to be at my ideal weight by then. Any tips or ideas for rekindling that fat burning motivation, or people in the same predicament?