Depressed

I feel like I have been depressed all my life.  I'm not sure what is wrong with me.  Anyone looking at me on the outside would think that I have it all together, admired by some, and jealous by some... if they only knew my tormented mind, what constant torment I go through that create in my own head...I have some great days but I have more bad days.  I try to mask my depression but I don't know how much longer I can keep it up.  I seem to have my highs surrounded by friends.  But I feel so isolated and alone.