Why can't I buy just one?

I have a problem that when I go shopping I have to buy every color of something, etc. Whether its a sweater or jacket. Especially if it is a good deal, even if I don't need it, I feel like I am going to lose out if I don't buy it. I also feel I will never see the item again if I don't get it then on sale. This has caused my draws to be overloaded and closets overloaded. The problem is I use to be able to return easily. Now I have a hard time returning something. It is very hard for me to bring something back. What is my problem? This has been very emotional for me. Especially when I know my receipt is going to expire and I can't return something after a certain date. I get very stressed between going and returning something for the money or keeping it. Its not that I need the item. Its just that it is on sale and I may see a designer handbag that retails for $300 on sale for $100. It is hard for me to resist. Then I feel guilty about buying something I don't need. I can't seem to make any decisions. I am paralyzed. Any advice is much appreciated. Thanks.