Hmm, I'm afraid I can't think of any way to downgrade again. If you had a backup, it wouldn't be too difficult but I understand you haven't. I guess you'll have to use an "emergency prodecure".
btw, you really should have put /home on a separate partition. It usually doesn't matter whether you do but right now it does make a huge difference. You would have been able to reinstall only / and leave /home alone. That would have been far more convenient. If you ever consider starting over from scratch, I recommend you do put home on a partition of its own: 7 to 10 GB for /, the rest for /home.
Now, the best solution would be to copy or move your data either to cd/dvd or to another partition and to start over. Considering you had an accident doing the upgrade, it would be a good idea to download a feisty cd and do a fresh install. It's radical but failsafe - and it may well be the only thing that will help you get a fully functional feisty.
If you want to move your data to another partition but efty is taking up all the space, you could download and burn a gparted liveCD. This will allow you to shrink your ubuntu partition and make a new partition in the freed up space. Be careful resizing: if you resize too much, you risk losing everything. Next you can move/copy whatever you need/want to keep so you can format the efty partition and install feisty. Just make sure you don't accidentally install over your emergency partition. And do remember to make a separate home partition. It needn't be big: you can always use gparted to resize it later, maybe after moving back your personal data.