The loss of stomach sleep seems to be a depressing milestone of approaching 30. I think I lost mine about a year ago - age 31. Up until a few years ago, the same thing - I was sleeping on my husband's ex -roommate's mattress of unknown age and questionable smell. I don't know why I waited so long to upgrade...
So congrats on apartment and bed! Who knows, maybe that mattress will restore your stomach sleep.
I guess there are more interesting things to use money on than a mattress. Travel, shoes, clothes, food...
I sleep on my stomach again, very comfortably so! I chose a mattress that is pretty hard, I think it's really good for my back :)