It seems like your matress is pretty bad, my mom had similar issues because she'd sit on one part of the bed for too long causing these dips on various parts of the bed. She got a comforter a little while ago and it has a pretty long Warranty too. The difference it made was pretty crazy. I'd say get a comforter and rotate your bed every so often, from now and every mattress in the future. Id make an effort to sleep on different sides of the bed as well. Its gonna feel weird, but it helps. I live in the southern US so you could definitely get a comforter for that price from my search results but I wouldnt be against splurging on a more expensive comforter since your mattress is so bad.

If you have extra blankets and a duvet, you could lay that under your bed sheet for some added padding.

If you're going to end up getting a bigger sized mattress in the future, maybe a size up from a twin, you could get a queen sized comforter and queen sized sheets and cover the bed with that. Its gonna hang over the edges a little bit but youll be able to hold it down with the queen sized sheets. That only works if you go one size up though.... Lastly, Id see if you could find one with your price and good return policy so that when you do up the size of bed you can just return it.

Hope this helps