Why is 3 stone so hard for me to loose? Please help!

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Well I'm 26 and currently a serving member of the Royal Navy and a overweight individual! I have let myself go and desperately need to get fit for sport and fit for work! Yet I can't seem to stick to my regime even though I am desperate and depressed about the current situation. I have a wardrobe full of lovely clothes that I can't fit into, I NEED ADVICE PLEASE X