Most likely, you're screwed.
That's what happens when you don't make more than one save. :shrug:
I think I finished my first playthrough with around 400 separate saves. Of course I would go through and delete old ones every now and then (which, in your defense, is a bit easier for me to do on PC than for you to do on 360).
The point is, you can't expect to play through a huge, open world game like NV overwriting the same save and not expect something like this to happen. You should keep at least 10 different saves, minimum.