wanting to lose weight

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I have probably been on over 30 diets my whole life, ever since I was 12 I started to notice that I was a bit chunkier then the rest of the girls in my school. I mean I was a size 12 then suddenly I was a size 14, 16, 18 & now I'm 20 years old and I'm wearing a size 22. I really want to get in shape and change my life when it comes to eating better. I wanna get fit and slim down for my wedding day, I also wanna be healthy and in good shape before I even get pregnant, I just always seem to start these things, but always end up giving up a few weeks into it. Can someone help me stay motivated so I can do this? :)