Hi spencerlempert,
Unfortunately there is really no way to fully correct support issues (that can lead to or aggravate back pain or discomfort) that are a result of a mattress that has too much soft foam on top or that has lost its support through foam softening … except perhaps on a temporary or partial basis. Anything you put on top of a mattress that either always was or has become too soft will only “follow” the softer layers underneath it and in some cases can even make it worse.
Having said that … there are a few suggestions in post #4 here though that at least may have a partial effect for a while and help you be a little more comfortable until you can purchase a new mattress (or do “mattress surgery” and remove the layers that have softened and replace them with higher quality and more suitable layers).
Phoenix