I have Hypoglycemia, sometimes my sugar drops in the mid 40's?

I can feel it coming the shakes, and need to sit & lower my head or I would faint. I have had this all my 68 years of life, My Dr. say my blood sugars are normal. But my feeling say different, I need to eat more often and if I dont I get the shakes,& become very light headed and then I don't want to eat at all,all I want to do then is lay down and curl up and sleep. Does eny one understand what is really low blood sugar?my levels of lows are in the 40 like 47 range, my highs are like 119 my highest ever was 129 I feel tiried all the time even as a young kid I use to pass out, But have not as i got oder say after the age of 20. Why is my doctor saying my sugar is normal when I don't feel like it is?