I've heard a nice explanation of the difference between learning English and learning German:
The rules of English are relatively simple but there are a ton of exceptions that take a long time to get right and will give it away you're not a native speaker.
German doesn't have as many exceptions so you can easily reach near-native levels of fluency once you know all the rules, but there are far more rules you need to be aware of (and even then you'll need to keep track of grammatical gender).
In other words, you can learn German by simply following all the rules and memorizing the vocabulary, but it's far easier to do for English (where you'll miss a ton of exceptions if you only do that).
Let's just say I'm happy I'm a native German speaker already -- I probably couldn't be bothered to become as fluent in it as I'm in English as a second language.
