Why I Am Not Able To Lose Weight Despite Yoga And Walks?

Hello Doctor!I was wondering why I'm having so much trouble losing weight. I'm 17 years old, 5'5", and 227 lbs. I work out every evening, (this includes some yoga, curl ups, sit ups, and attempts at push ups) and I walk almost everywhere. I eat plenty of fruits and veggies, when I drink soda it's diet, I drink plenty of water as well. But I'm gaining weight, still! Not in muscle either, from what I see. My family doctor told me it was probably a thyroid problem-- when I had blood work done, my thyroid count came out at 12! But the next time I went, it was down to 2. She could not provide me with an explanation for this. Could you?