I too had been deeply engrossed with reading for translation. When (or if) you start work on Lingua Latina, bad habits go away; but you really do have to start from the very beginning, where ‘Roma in Italia est’ is the very first sentence. I can’t remember who said that the first book looked too easy, that they already know all the stuff in the first book. I too ‘know’ most of what the first book has to offer, but it doesn’t matter, here’s why.
I have been learning linguistics on my extremely limited free time, and I came across something that best describes, mutatis mutandis, the problem I have with studying only for translation:
The great advantage of rewrite rules is that they are perfectly > explicit> . They do not leave anything to the imagination. By following them, you could produce a perfect English sentence even if you did not know any English, since the rules are applied mechanically, step-by-step, one symbol at a time. (Aitchison Teach Yourself Linguistics 2003)
Though I don’t translate ‘one symbol at a time’, this is essentially what I’ve learned to do, taking the Latin code which mostly I don’t understand and transforming it into the English language which I do (at least hopefully).
This does change with LL, where you learn Latin, not how to Anglicise Latin. See this article for an example of someone else who had already earned her PhD and still didn’t really understand very much without translation. She learned the real thing, eventually, anyway. What I’m trying to say is it doesn’t matter how long you’ve been learning one way; it does not mean that you can’t learn the other.
My advice is to pay very careful attention to everything Lucus has said in this thread. He voices the opinion which ought to be made general.
I look things up in the grammar books on occasion while studying from Lingua Latina, but I do it differently. That is, not to decode, but to get a better understanding for a language that I’m really learning in itself. Even when I’ve had to look up words in the dictionary, quickly they become Latin words instead of the Latin code for English words. Unfortunately I have had to drop LL until April 30th if I’m to do well at school, learning only from Wheelock, although I feel that I’ve passed the Wheelock stage; now it’s mostly just (re-)memorisation of what amounts to Latin code for English words.
LL does indeed give you traditional grammatical terms. However it only employs them only insofar as to help you with the actual passages. It makes no use of grammarspeak, which are things like ‘predicate nominative with a copular verb’ (or, among my favourites, ‘genitive of the sphere’! alhough I suppose by now its humorous value has been used up) and are wholly unnecessary to learn the simple concept behind the unwieldy terms. That’s what I think turns most people off to Latin; they have to learn a new meta-language at the same time as learning Latin. This quickly transforms into just learning the meta-language, and how to decode Latin into English. For most people, this leaves little room for actually learning Latin. I’m not sure how well I’ve expressed this idea, but it’s the best I can do for now in between studying for exams.
On the other hand, learning grammarspeak is tremendously helpful in modern times precisely because ‘the past century’s method has been to cripple Latin students’ at the universities; i.e., if I don’t learn grammarspeak, then I cannot (eventually) continue with my official studies in the language. Teachers expect you to parse even at very high levels (up through Masters classes at this university), and parsing uses grammarspeak. Also, the grammars I have use grand terminology like this. If I want to look something up, I have to know it in order to be able to make sense of what the grammar is saying. So far, I’ve learned grammarspeak well. It also helps in arguments on these boards, especially in the rare cases when people are just being nasty, trying to stifle the learning process by coming up with wholly irrelevant arguments that look important upon first glance.
Note that LL can be used incorrectly. For me at least, recording every chapter and listening to myself, as well as writing and recording my own exercises (based on the continuous plot offered in LL) on points that did not want to sink into this brain, were extremely important in aiding with fluidity.
As for my personal troubles (though I do not consider myself ‘more experienced’ yet; doing that, for me, requires learning Latin itself, something that since I have stopped by choice has been burning more and more ardently. Two weeks more):
-I too have problems remembering the adjectives and i-stems, specifically that neuter i-stems take the -i in ablative singular but m/f ones still take -e. Every once in a while, I put the -i on the comparative degree of adjectives.
-The passive voice of the present system still gives me trouble with the 2nd person singular (e.g. ‘caperis/capere’, but ‘capitur, capimur’ etc). It usually takes me a while to think of the right form.
-The participles of ‘ire’ are still mushy, as are ‘vis’ (the noun) and ‘nemo’ etc., and why does ‘domus’ flip between declensions?! etc.
) but, as I could easily fill a few pages, choose not to submit.
Let’s just say I don’t speak latin very well, and I’m not yet able to understand latin texts…But : labor omnia vicit improbus ! 
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