I can totally relate! I'm also 15, with my highest recorded weight having been 202 lbs. (I'm 182 lbs now, after less than two months.) For me, it wasn't so much the motivation that I needed - it was the knowledge that I actually COULD accomplish my weight loss goals, because I had tried before without success.
I've gained weight over the years for similar reasons. Winter holidays especially brought everything crashing down. I also have been fairly sedentary for most of my teen years, which hasn't helped.
First of all, what worked for me the most was simply convincing my mom to let me go grocery shopping with her. Personally, I have no trouble choosing healthy food at the grocery store...the trouble is when that package of Oreos is staring at me from the cupboard at home. Try new foods, too - I never
thought I would actually love kale!
Secondly, find your food inspiration. Healthy food blogs are awesome (especially ones with good pictures). If you don't know how to cook, now is a great time to learn; start with a knife and cutting board, and use Youtube to look up cutting technique videos. Eating well is 80% of the battle in losing weight, and being able to cook your own food at home is your best weapon.
Third, get to love the taste of water. I keep a 1 liter Nalgene bottle by my side wherever I go and refill it after every meal.
Fourth, identify why you overeat. For me, emotional eating was a major problem. I've done short periods of fasting a few times (~14 - 20 hours, which is simply eating an earlier dinner, skipping breakfast, and possibly delaying lunch). It helped me a lot to identify what is real hunger and what is just psychological desire to eat.
Fifth, get enough sleep (~8-9 hours) and practice stress-relieving activities and/or meditation.
Lastly, work on incorporating more exercise into your daily life. I want to start running, so right now I'm going for long walks and brisk walks interspersed with sprints. And I do a short yoga routine every night in bed to calm me down for sleep. Bike rides, dancing, swimming, basketball, kickboxing...whatever floats your boat, as long as it moves your body.
While I'm on the topic, I want to mention one more thing: food allergies and medical conditions. I have some kind of dairy allergy, so a long time ago I cut out all dairy except yogurt and the occasional bit of cooked cheese. Otherwise my throat swells up and I get bad earaches. However, it took me a ridiculously long time to realize that I have a bad food allergy to wheat products (I don't know yet if it's the wheat itself or some kind of additive/contaminant.) I get skin rashes, headaches, tiredness and brain fog about 20 minutes after consuming it. Once I cut out the wheat, the weight finally started dropping off. Make sure you don't have any allergies or medical conditions that are preventing weight loss.
But don't try to change everything drastically all at once! The best diet and best exercise routine is the one you can stick with.