Okay, one, you should NOT be riding a horse in that condition. Two, you definitley should post an up to date picture. As others have said the picture you posted shows an extremely uderweight horse, that doesn't just happen overnight, sorry.
Has she been checked over by a vet? Age? Current picture? Any known issues?
There are so many questions left to be answered here, please let us know!
At this point though, like cowboybob said she needs a vet. From the picture you posted she really should have some vet attention, whether to check her over for an underlying issue, teeth problems, general pain, etc...
I don't know what else to say.