How far from the TV are you sitting? 37 inches is good-sized, but not huge. If you sit farther from your TV than the average person, it's very likely that higher resolutions won't be noticeable to you. There's a famous chart floating around the internet that will give you a rough idea of what I'm talking about. I'm not sure how accurate it is, but I've never seen it specifically denounced, so I think it's decent. Anyway, according to that chart, you're likely not to notice HD at all at distances around 13-15 feet (lower than that for 480p, but SD is 480i, so I'm guessing 13-15). That's not an excessive distance if you have a big living room, maybe have the TV above your fireplace or something, etc. Besides, even if you're a little closer, the difference may be marginal.

Other possible factors:

1) Cable "HD" is not always the best quality, and can vary by program. Put in a blu-ray for an accurate test.

2) TVs have processors that handle your source material before it's displayed. Your TV may be particularly good at showing SDTV, and perhaps not great at HDTV, making the difference more marginal.

3) No offense meant, but your eyes may genuinely not be that great. *shrug*

If I were you, I'd stand 5-6 feet from your tv, and find a program that's playing on both HD and SD channels at the same time. Sports are ideal. Flip back and forth between one and the other. If you see only a very marginal picture difference under those circumstances, you probably need your eyes checked. If you see NO picture difference (other than brightness or whatever) then something may be wrong with your setup in some way.