I am working through Wheelock on my own and can share in your frustration. One thing that has helped me immensely, especially with nouns and verbs, is doing practice problems. I've used just about every resource I can get my hands on in order to apply
the paradigms. Sure, I still slip up sometimes, but the more you practice as you progress through Wheelock, the more these paradigms will stick with you, even if you don't have them fully memorized by the end of the chapter in which they are presented.
I'm posting a blog as I progress through Wheelock with some notes and my experience. Please feel free to browse through it.
I share the opinion of others here, that Wheelock's treatment is very terse. I think that using the chapter alone, even with the self-tests in the back of the book is insufficient for lasting comprehension of the material. This is why I've used it in conjunction with other resources (many of them are available online for free). Practice makes perfect.
Best of luck to you,
Here is a recent post on my strategy for working through Wheelock:http://polyglotte.tumblr.com/post/7111063699/wheelock-review-of-course-plan