> Holds also for Portuguese (in Brazil)

Brazil was a great place to practice Spanish. Almost nobody spoke English when I visited, so I ended up speaking Spanish with everyone. Some people spoke Spanish themselves, and for the rest, the mutual intelligibility with Portuguese was good enough to get by.

I can speak Spanish, and I find that Portuguese is really hard to understand. In written form I can usually figure out what the meaning is (as I can do with other, more distant languages such as French or Italian), but hearing it spoken, I can’t make sense of what is going on at all. It sounds like Spanish, but all wrong, and I find that I just can’t focus on trying to parse what the words mean.

You probably mean Continental Portuguese. Brazilian Portuguese is really much better for learning. They don't eat syllables and they take their time to pronounce and emphasize words. You still need to know a few rules but they are sufficiently regular to eventually start guessing words (e.g., "ue" within a word -> "o", ending "dad" -> "dade", ending "ion" -> "ão").

Source: I've had at least half a decade of frequent contact with both dialects and I still have way more difficulties with the continental version.