challenge, probably b/c I began learning aurally when we lived there and I have a hard time abandoning the phonetic spellings I imagined as I was learning.
I couldn't agree with you more about diagramming sentences! Hated it when in school, but it really is the best way to make grammar an ingrained part of ones language skills. I'm not sure I could still tell you all the rules, but I can just "hear" when the grammar is wrong in a sentence.
As for help with fluency, have you tried watching movies in Spanish with the English subtitles on and vice versa? Alas, I can't do this much in the States b/c Italian isn't a common subtitle choice here (used to rent flicks w/English subtitles in Italy), but you should have lots of options w/Spanish. Reading one language while hearing in the other really helped me to get comfortable with phrasing. You might try Spanish language soap operas, if you have those channels w/your TV service. Soaps feature lots of repetition and simple, universal plot lines. And, of course, there's youtube.
Good luck!
(and know that if you ever go to Italy, you can speak Spanish there and probably get your message across about 75% of the time! )