I've been studying Latin for about 15 years as a hobby. I remember about 10 years ago, I picked up a Teubner edition of the Histories of Tacitus. Confident and cocky after a few Latin courses, a couple textbooks, a fair amount of reading from abridged anthologies of Cicero, Caesar, etc., I opened up the first pages of the Histories. I understood nothing. It came as a shock, a terrible blow to the ego. I dismissed Tacitus as an oddity, reassuring myself that my Latin was reasonably good, and went on with life.
My Latin progressed at a snail's pace. At this time I was studying law and working part time, so I didn't have much time for hobbies.
A few years later, I decided to improve my Latin. The only way to become fluent in a language is by exposing yourself to it. With a dead language, that means lots of reading. So I read a lot. I read most of Cicero's speeches, and to alleviate tedium (how's that for Latinate English?), I also read some translations of modern novels, such as Treasure Island by Arcadius Avellanus.
Eventually, I picked up a copy of Livy's Ab Urbe Condita, book 1, that had been laying in my bookshelf for a few years. I had read some excerpts of Livy before, and seemed to remembered that he was neither easy nor hard, but reading straight Livy with no notes was more difficult than expected. I drudged through book 1, and when the time came to get book 2, I decided on a bilingual edition. This helped me get the hang of Livy, and after a few books I stopped looking at the translation altogether, referring only to my dictionary. The more I read, the easier it got.
That Teubner edition of Tacitus's Histories was still sitting on my bookshelf mocking me. So I gave it a go. It was very difficult, but manageable. Two things gave me the ability to read Tacitus: First, my Latin had improved a lot. Second, I had developped an uncanny ability to "expugnate" or take a passage by force. It's hard to describe. It involves staring at a passage for 10 minutes or more, looking up words you think you know, but then realizing that a shade of meaning in this particular context is the key that unlocks an obscure passage, reading ahead to get clues as to the meaning of a previous sentence, and more generally thinking outside the box, being creative to guess what the author could mean. This is how I realized that reading Latin is as much a question of practice as it is of knowing vocabulary and grammar.
Tacitus is notorious for omitting implied words and being so brief and concise as to create ambiguity. But he is a pleasure to read. His style is unique. His is a different kind of eloquence than Cicero, I would say more sophisticated, less polished.
I challenge you to take the Tacitus test. If you can read several pages with the help of a dictionary but without referring to a translation, then your Latin is at least as good as mine. If not, then you need to read lots of Latin from other authors. I suggest Livy because it worked for me. If you manage to read the first and third decades (books 1-10, 20-30) of Ab Urbe Condita, your Latin will improve drastically and as a bonus you will learn a lot about Roman culture and history from Rome's greatest historian, which will help with other authors.