PLEASE HELP-"GOOD MONTHS-BAD MONTHS

I have been experiencing this for years, its just recently starting to get worse. I have what I call "off months." I will go sometime with feeling ok...tired a lot but able to focus and live my life. Then--there are my "off months." They are TERRIBLE. Im EXHAUSTED almost ALL the time. I work out 4 times a week, for a minimum of 45 mins (carido, etc at the gym) eat pretty well, drink only maybe 2 times in a month (a drink or too, never any binge drinking) I DO NOT DO DRUGS and DO NOT have any kind of substance abuse problem. I have hypothyroid, and take medicine for that, my levels are checked often and are fine. I just get into these modes where I feel, BLANK. Its the only word I can find to best suit how I feel. Im tired, but still push through my life, things need to get done-I have 2 kids and a career. During my OFF months, I have the HARDEST TIME concentrating. It takes so much to retain information, and focus-when Im driving it takes DOUBLE effort, and it almost feels like im "dreaming" some of the time...I have a great life, not a lot of stress and a stable household. I try to "ignore" these months and "push" through them. The biggest complaint is my inability to retain information, I get distracted and feel like Im almost "dying" because Im so "worn down." What is going on here? I have seen therapists/psyhcologists and they want to put me on antidepressants, I took one before and it was TERRIBLE--I took it for 3 months and the side effects were AWFUL. I dont think im dpressed, could it be manic, or a "touch" or bipolar?