Your acquisition skill was not lost. You were taught a simplified version of language that treats vocabulary and grammar as separate storage units, but real acquisition happens when your brain maps sounds, meanings, and patterns together through use. The "-er" and "-ly" trick is useful for quick identification, yet it is only a clue, not the mechanism.
Children learn grammar through enormous amounts of repeated, meaningful input. They hear thousands of sentences before they produce correct ones. Vocabulary and grammar also do not separate cleanly. When you learn a word, you learn its behavior: which words it combines with, its endings, its position in a sentence. That behavioral knowledge is grammar. So the real question is how to give your brain enough usable data.
Adults can absolutely recover that acquisition instinct. The most reliable method is comprehensible input, meaning content you understand about 80 to 90 percent of. For Dutch, start with beginner podcasts that have transcripts, simple YouTube channels about daily life, or graded readers. Listen and read the same piece several times. Do not pause constantly to analyze. Your goal is to experience the pattern, not to prove you know it.
Explicit grammar still has a place. A short explanation of Dutch word order, separable verbs, or gender agreement can save hours of confusion. But treat rules as checkpoints, not the path. After reading a grammar note, go back to input and find real sentences that use it. That is what makes the pattern stick.
Your old school method grouped words by endings: "-er" nouns, "-ly" adjectives. That is a legitimate shortcut, and Dutch has similar regularities. You can use them to guess part of speech, but do not let that become your main study mode. Instead, collect whole phrases. For example, learn "de leraar" with "de" and a sample sentence, not as an isolated noun.
A practical day might look like 15 minutes of listening, 15 minutes of reading, 5 minutes of sentence mining, and 5 minutes of review. Use spaced repetition software with sentences, not single words. After two or three weeks, you will feel slower, but that is normal. The skill is rusty, not missing.
The honest tradeoff is that pure input is slower for adults in the short term, while pure grammar feels productive but fades. The best approach combines both with heavy input. It need not be expensive. Many free and low cost resources exist. Evaluate any course by whether it gives you hours of comprehensible audio and text, not by how many rules it lists. You want material you can understand and enjoy, because interest is what keeps you listening.
So no, you have not permanently lost the acquisition skill. It is still there, hidden under classroom habits. Feed it with meaningful Dutch, use patterns as clues, and let your brain do what it has always done.