You probably aren't an idiot, it's just that language learning is harder than it looks like.

If you have a really hard time with the nasal vowels, worst hypothesis it's fine to simply pronounce an oral vowel + [ŋ]. You won't sound native, but it sounds less off than simply omitting the nasalisation or using [m] or [n] instead.

Watch out for the vowel height of /ã/; depending on the dialect it's between [ɐ̃] and [ə̃], but it's certainly not [ã].