How high does your blood sugar need to be to get symptoms?

I was recently diagnosed with pre diabetes because I went to the doctor having all the main symptoms of diabetes. I had lost a lot of weight in just a short amount of time without trying. I was hungry all the time, but my stomach hurt. I was SO thirsty and I had to pee pretty much every 20 minutes. My doctor did the fasting glucose test and the A1C and said that my levels were close, but not quite diabetic yet. She also said that since my dad is diabetic and he has a meter that I should test periodically and if I start seeing numbers consistantly over 120 that I need to be retested. The reason I'm asking about symptoms is because someone I was talking to told me that unless your blood sugar is like 400 or higher you won't have any of the symptoms I had. I haven't tested very much, but I have a few times when I started to get the familiar symptoms and my blood sugar was pretty much always over 120. 2 of the times it was 150 5 hours after eating, but the next morning I checked and it had gone back down. So I was wondering if this friend was right or not? Does you blood sugar have to be REALLY high to have symptoms or does even just a little over 120 give some people them? Also, because it's been high so often I find that I feel sicker when it drops back down again, even though I know it needs to be lower. Is there any way to fix that? I already eat really healthy, and I've cut out basically all sweets except on rare special occasions (like Christmas! Haha) Even when I do have them though it's just in small portions. I'm an average weight, and I've started exercising more so that my body would need the carbs and sugar instead of having too much. Is there something else I need to be doing along with that? Does the fact that I'm doing everything my doctor said and still getting the high numbers mean that I'm going to get diabetes anyways? Please help! :)