yeah. you have a rather large problem on your hands.
you are addicted to stuff.
now, i know this is going to sound totally off topic, but, please bear with me :D
do you have a religion?
you're probably shopping so much because you don't feel like there's any purpose to your life - a lot of people do that - they feel like, if they dropped dead tomorrow, no one would really care. if you do feel "worthless," i want you to know that God loves you and has a purpose for you if you'll accept Him *gets off soapbox* not tryin' to write a sermon, but it's true!
try this: everytime you get the urge to shop/get depressed, get with a friend. go do something fun (other than shopping) like going to the park and feeding the ducks or going to the fair or even having a water balloon fight. something totally immature.
also, try "adopting" a child from peru. there are tons of charity orginazations out there that charge you about forty (or less) dollars a month (i'm sure you can afford that, since you're spending waaaaay more now) to clothe, educate, and feed the child. you can even send the kid stuff! so, if you catch yourself in a store, look for something to buy your little friend. and leave.
if you don't have the willpower to just leave, get a friend to go with you that will force you to leave (don't you love those kinds of friends?) work on it until you can make yourself leave.
yes. you do have a problem, but, sadly, so do thousands of other american teens. the stuff i suggested is not only to get your mind off your possessions, but to get you involved with friends, family, and the community. that's what's really healthy for a teen to be doing. :D
a system that always works for me: whenever i get the urge to shop and i know that i don't really need anything, i open my closet and lay all my shirts out on the floor. i try each one on and scrutinize. if i don't like the way i look in it anymore, i give it away or make it into something new. then, i move onto pants, etc. usually, there are tons of clothes that i've forgotten about and so, to me at least, it's like a free shopping trip!
something else that's really fun to do: have a clothes trading party with your friends (preferably, the ones that are about your size in clothing) have them bring a bag full of clothes that they don't really want anymore and trade with them. if you get a lot of people to come, it's a blast because there are tons of cloths for you to "shop" through and everyone wins (even your parents' checking account)
hope you overcome this! i know you can :D