Glucose toxicity refers to the oxidizing and hypertonic (dehydrating) properties of hyperglycemia, both of which continually stress and damage tissues in the body. But the term is specifically used to refer to the phenomenon of temporary insulin resistance brought on by this tissue stress.
When a diabetic animal is hyperglycemic for long enough, the animal's damaged tissues start having trouble using insulin. This in turn means that even a well-dosed animal will continue to have high blood sugar, leading to even more insulin resistance.
Various ways around this "glass floor" have been tried, to varying degrees of success. Unfortunately none of these methods is particularly safe. One way is to continue gradually raising insulin dosage until the tissues pick up the insulin and start absorbing glucose, then quickly back off to a lower dose. Another is to "jumpstart" the process with a fast-acting insulin or a deliberately high dose of the regular insulin, then quickly back off to a lower dose.
Since either method risks overdose and hypoglycemia once the "glass floor" is broken, this problem is best regarded as dangerous and should be addressed in close partnership with a diabetes-experienced veterinarian.