When your sick do you go to the Dr. and does he/she put you on an antibiotic? Some Drs. overdo with antibiotics and our system just rejects them after awhile, especially if they are the same ones. If it's a viral thing an antibiotic isn't going to help anyway. You also have to remember to finish the entire doseage or it will only come back stronger.
Along with cutting way back on sugar have you ever taken a Probiotic Acidophilus? My Dr. put me on them a couple of years ago while taking an antibiotic due to getting Diarrhea but I take one almost everyday to help my immune system, so far this year I have done fairly well with just one cold.
What kind of blood work has your Dr. done to rule out anything serious? Hope you get to the bottom of it all, as it sounds like your immune system is very run down.
Best wishes and feel better...JJ...