It appears that you may have some structural damage to either the rear pan or the bumper support, or both. What does the right side look like? You're probably going to have to remove the bumper cover to see what's bent. But I'm sure something is bent. Has the car been in any accidents that you know of?
It's pretty easy to remove the bumper. You just need to unplug the license plate wiring, then remove the phillips screws at the front of the bumper cover, inside the rear of the wheel wells. Then there are four nuts holding the bumper support to the car that you will need to remove. Two of them are accessed from under the car, and the other two are behind the rear panel cover in the trunk. Here is a link to the online factory service manual that shows where the fasteners are. Yours doesn't have the factory rear under spoiler so you can ignore that part...
http://media.honda.co.uk/car/owner/m...f100/14-51.pdf
That's a nasty rust problem you have going. I'm wondering if maybe there isn't a rust problem causing your bumper alignment problems as well.