One of the biggest lightning bolts I ever had was when I realized that it just didn't matter what other people could eat/do and stay slim. They have their body, I have mine. I have to work with mine, that's all I've got. I can't change it, and I certainly don't have time or energy to spare to devote to worrying about it. Every minute I used to spend thinking, "Why me?" and wishing I were them was a minute wasted, not only in the cosmic scheme of things but in practical terms.
We are strongly influenced by the way we talk to ourselves. When you say negative things to yourself, railing at the universe for the unfairness of it all, not only do you waste time but you are teaching yourself hopelessness. You simply reinforce all of the thoughts that hold you down and make you feel no effort is worthwhile. You convince yourself that because you weren't born with all the cards in place, that your entire existence is doomed.
That's not true, is it? Of course not!
So, every time that Evil Voice looks at your slim friends scarfing down cheese fries, remind yourself that you don't stick with them 24 / 7. They may compensate for those fries later today or tomorrow or later in the week. Even if they don't, getting angry and discouraged because they have a different metabolism is pointless. This is the same infuriating behavior that causes women to fixate on others who have large boobs or blonde hair or round butts or white teeth or high cheekbones or ... whatever the media has taught us is desirable this week. First, you don't know whether they were born that way or if what they've done to get it, but in the end it doesn't matter. Second, rather than spending time and effort making your brown hair look as nice as it can and taking pride in its native lovliness, you wallow in jealously and negativity.
So, head up, be proud of what you have accomplished with YOUR body, and make peace with the fact that you may have to work harder than some others, but that that's just the way it is and you'll do it because you want to be the best you that you can be.