You are not alone. Many really do spend too much. Buyer's remorse is extremely common, even for pretty good purchases.
I have hundreds of computer games sitting here yet to be played from years of buying that maybe I could use a bit more remorse about - an d I still buy more. It's almost as if the fun is in how they look to play and having them available, it's odd. I rationalize it as 'supporting the art of computer game making', in part. 
Part of it is simply being around too much marketing. As I browse hot deals and bragain web sites you pass up hundreds of deals and then you see a big sale and...
So I sit here with many components of PC's yet to assemble and more hard drives than I can use. The 200MB 8MB one for $20 last week was particularly useless but 'too good a deal'.
On the other hand, I tend not to buy more expensive things I should, like a decent car; I had an old nasty mattress over a decade too long, an old small tv a decade too long...
I finally got a nice new Sceptre 42" LCD for $2000 last June, now it's down to $1,300 with a second HDMI port added, and the Costco return policy is calling out to take advantage...