Sometimes we can develop an immune reaction to non-human insulin. All the modern analog insulin's, including Levemir and Novolog/Humalog are non-human. Our bodies can generate an immune response, generating antibodies to attach the foreign insulin. These antibodies are called anti-insulin antibodies. This is different than anti insulin Auto antibodies that are a marker for T1. There is a test for anti-insulin antibodies. If you are generating a lot of these antibodies, they will deactivate your insulin and the result will be insulin resistance.
But I have to tell you, taking 8 units of Levemir a day and have in ICR of 12-15 does not indicate insulin resistance. Unless you weigh 50 lbs, that indicates you are actually insulin sensitive. If you took 200 units /day, then you might worry about insulin resistance.
Personally, I hate the term brittle. It implies that your blood sugar swings wildly in an uncontrollable manner and without explanation. And it carries a stigma, that you are not doing a good job of controlling your diabetes. You may well be having wild swings. It is entirely possible that you have gastroparesis as others have suggested. But I actually believe our blood sugar always swings for a reason, even if we don't as yet understand the reason. And gastroparesis can slow your stomach from emptying properly and hence you bolus for a meal and the meal is delayed getting digested, and blammo you have a hypo. And then whey your good finally moves, you go high, 2, 3 or even 5-6 hours later. This can lead you to believe you are insulin resistant, but your aren't.
Dr. Bernstein recommends the use of the R-R interval study for diagnosis of gastroparesis. The vagus nerve controls a bunch of things, including your heart and digestion and when the nerve becomes damaged over time, you get gastroparesis. The R-R interval study simply looks at your heart rate difference between inhaling and exhaling. According to Bernstein in Diabetes Solution, "If, during an R-R study, your heart rate varies only 28 percent between inhaling and exhaling, then you will likely have mild gastroparesis. If the variation is about 20 percent, gastroparesis will probably be what I call moderate, and if less than 15 percent, I would call it severe." The R-R study can actually be performed by any doctor (including your GP) who has an ECG machine.