Getting stuck on vowel length—how do you hear the difference?

I’ve been recording my Dutch audio for weeks, and looking at the waveforms, I still can't tell if I’m hitting the long vs. short vowels correctly (like 'man' vs 'maan'). I keep getting feedback from my exchange partner that I sound like I'm 'swallowing' my words. Are there specific minimal pairs or exercises that helped you internalize these lengths so it doesn't sound like I'm just guessing?