Couple of things - think of depression like diabetes - it might very well be a chemical imbalance in YOU that you will need to take medication for the rest of your life! Remember how good you feel when you first went on it...that's the way it's supposed to feel all the time. And Lexapro might not be right for you - it definitely wasn't for me. It's taken almost 3 years for me to get the correct dosage -- I was a bad patient! - hated taking meds and as soon as I felt better, I'd stop. Boy that'll screw you up fast!
Two - you know the cycle of negativity, you mention it in your post -- but you know what, the positive cycle works the same way. Kind of like weight loss - you diet and exercise, you lose weight, you diet and exercise some more, you lose more weight - and it continues to cycle. Think of coming out of depression as the same thing - you feel better, your grades improve, which makes you feel better, which makes you try harder and your grades improve more. Remember, you are your own worst critic...hang in there and vent here anytime, even just reading the posts helps me sometimes!
