I find that different people can manage different things. I'm a knitter and can't crochet for toffee. One of my friends can crochet but not knit. Another of my friends can't do either but is the neatest cross stitcher I've ever met (I mean she gets the back looking almost exactly like the front).
If you've not managed to get the hang of knitting or crochet through all the beginners books, videos, one on one help then chances are knitting and crochet isn't for you.
Have you tried other things such as quilting or working with felt? Perhaps go a different tack and do something paper based such as quilling or making your own notebooks/scrapbooks etc.
There are so many options when it comes to crafting so don't feel too disheartened by your lack of success with knitting/crochet. You might have hidden talents in another art form.