Language learners run into this the moment they try to use what they have studied: the reply arrives in English before the sentence is finished. The switch is easily read as a verdict on the learner's Dutch, which is what makes it discouraging enough to stall practice altogether.
It's not a judgment on your Dutch, it's a time calculation. Dutch culture runs on efficiency and low tolerance for friction, and to most people a halting conversation in your second-best shared language is slower than just switching to their (excellent) English. It happens automatically, often before they've even registered doing it. To counter it, say so explicitly and early: "Mag ik oefenen met mijn Nederlands?" (can I practice my Dutch?) works because it reframes the exchange as a favor they're doing you, not an obstacle they're navigating around. It helps to pick low-stakes settings first — the bakker, the markt, small talk with neighbors — where a slower pace doesn't cost anyone much. Taalcafés (language cafes, often run by libraries or volunteer organizations) are built for exactly this: everyone there has opted in to speaking slowly and correcting you. Repetition with the same people also matters more than raw hours of study; once someone's used to your accent and knows you want the practice, they stop reflexively switching back.