Hi L.-
One of the ways my son's AD/HD manifests is VERY similar to what you've described. I would certainly speak to your pediatrician about it. It's really frustrating because kids with ADD or AD/HD are able to focus on certain things, and not others. It may not be her fault, and you are right to hold her accountable, but maybe you can find some ways to be proactive.
If I could make a few suggestions, I would start with Dad keeping his own set of clothes for her. It didn't work for me, but my sister's ex does, so you might try it. After dealing with missing clothes for too long, I always wrote out an index card with what I was sending over and stapled it to the bag. That put Dad on notice that I was keeping track, and it made it very easy for him to say "do you have your striped shirt?" or check the bag himself. And don't send her in stuff you will miss if it doesn't come home (been through THAT).
I know she's 11 but label the heck out of EVERYTHING, like you would if you were sending her to kindergarten or summer camp. That way you are enlisting the help of teachers, bus drivers, etc. without singling her out that she's losing stuff. They'll find it and just naturally return it to her. You can run off a set of computer labels very easily and just start sticking them.
Also, make sure to help her keep REALLY organized. I keep a bin by the front door with only ONE set of hats & mittens per kid, that way you know quickly if they are there or lost. Chances are good you'll have a stash of spares, but keep them someplace else, or she will rely on you to just replace them all the time without looking around first. I would scale her room down, too and again label EVERYTHING. It will help her to keep herself organized if she knows exactly where everything goes and where to look if she can't find it. Also, too much stuff is really chaotic for kids in general, I think.
Also, every night, together check her backpack and make sure it is REALLY organized and keep the extra stuff down to the bare minimum.
With this kind of problem, and you being a single mom (again...been there!), you really have to choose your battles, so if it means that she doesn't get a cool lunchbox anymore and has to use brown bags, so be it. I used to have my son color on and decorate his bags while I made dinner, it kept him busy and made the bags a little more fun.
Also, I know it really matters to girls her age, but take an inventory of how much stuff she really has, like shoes and bags and hair stuff. If you took a Saturday and together picked out say 3 sets of each to keep out and put the rest in a bin in YOUR closet (or whatever, just not where she has access to it), then she can use those for a while and when she wants a change you can "go shopping" in what she has and make a one-for-one trade.
I hope this helps! Again, I would check with the pediatrician just to be sure there isn't something else going on. If it's causing upset in your household and you've done everything humanly possible on your end, ask your Doc.
Best of luck-
Jen