I wholeheartedly agree with the previously expressed by Judy. There´s the influence of what we have learned before, our vocabulary, background, even the anxiety to learn (and don´t show our weaknesses, or the need to communicate right at the moment).
Somehow we don´t see children stating that some point of their mother language is difficult, that it sucks, or similar statements.
From the teenager days on, they (we) learn that Maths, languages and other subjects are difficult, they suck, etc. They know then, that they have a challenge and that learning their language is a fact of life, and they learn it, or at least enough to to communicate.
It´s not our fault (lame excuse, but it works...), because we learn a code (phonetics, pronounce, the letters and even the "register" of the sounds.
Then we are presented a new code, we immediatelly to reverse to the previous one, that´s understandable.
Training, practicing is the antidote against the aquaplaning here, since we instinctively try to adopt aspects of one language into another, to transfer the knowledge of one to the other. This is perhaps one of the first and what peeves the new learner the most.
Interesting and funny, is that the learner of English has the same difficulties with the grammar (with little variation). The questions make a pattern of sorts, even when the student is a Chinese, Brazilian, Spanish and from some other countries!
Again, I agree with Judy when she says that in the scale of difficulty of languages we are in the same level, taking into account that the learner from abroad will instinctively to use the code he has learned to communicate/write in Portuguese, and vice versa.
Not to mention that learning a language is not only "the code itself", there are things that are embedded into the culture (of the country of the target language and that of the learner). One Brazilian (or European) that is used to say that a cat has seven lives, will have to add two more in the English-speaking countries (where cats have nine lives).
And before taking a picture, one will ask to say X in Brazil, there he/she will to ask to say cheese! So, that doesn´t have to do with grammar or teaching, to a degree. That´s why the learner must use a bit of independent learning, find English everywhere too, not only the one from the lessons.
Plus, English is available everywhere, in music, internet, technology, in the radio stations, TV, whereas Spanish, French or German - to name a few, isn´t so ubiquitous. So, they will be harder to learn. The immersion factor!