I got my hair chemically straightened, but it didn't really work. What do I do now?

I'm going into my senior year in high school. I have gotten it done about twice a year since I was in eighth grade. My hair is not as straight as it has been in the past. I've gotten a keratin treatment on it, too. It worked, but only lasts about 2 months. Then I got it chemically straightened just about 3 weeks ago. I'm upset because it's really not that straight. I know it's 'bad' for my hair, but my hair is sooooo THICK. :/ Could it be that my hair is somehow immune to it now? What should I do about this? If I pay over $80 to get this done and it doesn't work, obviously I'm upset. Thanks for your help!