108 is not crazy high, and neither is the 158. But you don't want to be running numbers that high on a regular basis, of course. There is definitely hope that if you follow the diet and get good exercise you can bring the numbers down on your own, but if it doesn't happen, please don't beat yourself up. Fasting numbers especially are very difficult to bring down with just diet and exercise -- it can be done, but they aren't directly affected by the meal you just ate, unlike your post-meal numbers. Fasting numbers are more a reflection of your background insulin resistance, which during pregnancy unfortunately tends to just keep rising. It's because your placenta is producing hormones that help increase insulin resistance, and not because you are eating poorly or have "really bad diabetes" or whatever. That having been said, some people are able to just follow the diet + exercise and keep their numbers in range.
I agree that testing religiously after meals will help you identify your "trigger foods" and help you know what to eat and what to avoid. Everybody's body is a little different, although in general you can't go wrong sticking with low-glycemic foods and making sure to eat a lot of protein along with your carbohydrates. Exercise after meals also tends to help me quite a bit. A good brisk walk, or 20 minutes on the exercise bike, can drop my level by 20-50 points (sometimes more).
A lot of care providers are revising their GD protocols these days, and I am hearing from more and more women whose providers didn't start talking about induction until 40 weeks, or even later, which is obviously a huge improvement from the days of automatic induction at 38 weeks. Unfortunately, there are still some providers out there doing the automatic induction at 38 weeks thing, even though there is really no good reason for it. If it comes down to the wire for you -- which hopefully it will not -- my advice is just to use stalling tactics and perhaps go in for a few more monitoring tests (non-stress tests, etc.) than you would otherwise. If they say, "Well, we want to induce," ask why. If the baby is looking healthy, your blood sugar is in good control, and there are no other contraindications such as high blood pressure, etc., there's no reason you shouldn't progress normally with your labor.