My house is bad, and I'm 26! :) Thankfully it is mostly just toys, books, boxes (the kids play with empty boxes so it looks awful but they have fun), and pillows and cushions they are playing with on the floor. However, I had a very messy room as a kid, and that was not that long ago.
I struggled to be neat--it did not (and still doesn't) come naturally. Some children and adults are just naturally neat and clean. I usually had better things to do (like reading or playing outside) and at the end of the day, my room was not a priority. I also cleaned house and was starting to mow the lawn by that age, so I was doing a lot of work and again, my room just was not a priority.
It is reasonable to talk to your daughter about neatness in her room. It is as much for her health and safety as your sanity. You could set out some ground rules for her room (she lives in it--it is not going to be perfect), talk to her about what would make it easier (like rearranging, buying some organizers, helping her sort), and make a project of it with her. If there is something that would make her want her room to look nicer, use that as incentive.
My bedroom got redone when I was a teenager (it had never been done--it was in an old farmhouse), and having my walls painted, a new light fixture, my bookshelves built in (a shelf above the windows or near the top of one wall can be pretty, too) made me want it to look nice. I also struck a deal with my mom (again, I was a little older at this time) so that I could have candles in my room but only if I was keeping it perfectly clean. I loved the smell and wanted to be able to light them, so I kept it pretty neat and enjoyed seeing it clean. Take pictures when you have it arranged because she might really not "see" it the way you do.
Anyway, you are not a bad mother because your child's room is messy. Everyone's housekeeping standards are different and children have competely different priorities. I hated having a messy room as a kid, but I just couldn't bring myself to spend the time cleaning, or if I did I got so distracted putting a book away I'd sit down and read it. I actually liked it when my mom came in on a Saturday and sat down and talked me through it. She would tell to clean up the books, for example, then the laundry, etc. I was so distractable but she would keep me from realizing I'd spent an hour standing somewhere reading a book. Set reasonable standards and talk to your daughter about how to get that done. If cleaning is too boring, I set a timer sometimes or turn on a fun song and make myself focus on doing as much as possible in that time. She might never be perfectly clean and tidy, but you can help her be a bit better.
I also wanted to say that if she is keeping her room clean for five weeks straight, that is better than I can do with my house. :) The soda cans would be a problem for me--it would help to make a no snacking in the bedrooms/only drinking water rule. This also helps in general if snacking is designated to the kitchen and dining areas. I also wanted to say that if your daughter's habits are kept in her room, relax a bit more. If it is not creeping down the stairs and taking over your house, you're doing pretty good. I got better in college because I had only ONE room to clean instead of a whole house, and I cleaned and mopped it obsessively. My roommates were another story... :)