What mattress topper will alleviate my back pain?

I am 18 years old and have been suffering from back pain for over 2 years. We have had tests run and they show nothing that can be treated in my back. We switched my mattress with a firmer one last summer and that has worked until a month ago. Now I'm going to sleep and waking up with backpain so intense that I cry. Is there any kind of mattress topper that I can add to my firm bed that will help alleviate my bak pain. My parents basically refuse to do anything. Please help me.