I bought a mattress around 3 years ago. It is so painful to sleep on it lately. The whole bed is sunken in after such a short amount of time. What can I put on top of the painful spots?
You need more than a "cover" as generally a cover is relatively thin and is used to protect the mattress from staining.
What you can try is getting a reasonably thick memory foam mattress topper that is 2" thick at least - 3" is even better. It should make your mattress more comfortable for a bit longer.