In need of motivation help please?!

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Hey everyone! I'm a high school senior who is trying to lose 35 pounds before prom and for life in general. I am almost at my thirty pound weightloss but I am slowly losing motivation and I don't know why. For some reason I worked so much harder in the beginning and now that I'm increasing my speed on the treadmill I can't seem to run as much as I used to because its harder. I am losing confidence in myself and I'm so close to my goal I just need some motivation. I have started a lifestyle change in September and my weight has been yo-yoing profusely.

I just want to be confident in my OWN abilities not anyone else's how can I do this? Thank you all!