OP, get another mattress ASAP. I was in a somewhat similar situation with an old, sunken mattress and it effed up my back (my back is still messed up 7 years laterĀ smiley: ohwell). The old mattress was in my room at my parent's house and since I was away at college and only visiting once a month, we kept putting off getting another mattress. It got to the point where I'd have bad back pain when I woke up but as the day went on, it would go back to normal. One morning when I woke up , I was having bad back pain as usual and then moved the wrong way to get out of bed. I was in excruciating pain for 2 weeks after that, then moderate pain for a while, and now I have a dull pain that's constantly there and that gets worse every once in a while.

As far as the whole full vs queen debate: I've had full mattresses since I was little and finally upgraded to a queen a few months ago. Around here, the queens were about $50 more. I really don't notice much of a difference in terms of space since I keep to one side of the bed anyway and don't really even move much when I'm sleeping.