Why can someone understand a language but can't speak it?

Before I went to college, I could hardly speak Mandarin. Foreigners should be able to understand what I say in Dandong dialect. However, when I was in college in other places, I was laughed at countless times for speaking dialects, so I learned to speak standard Mandarin from a painful experience. I can only speak a few words of Mandarin at first, but I can't help but change back into a dialect. But I really made up my mind at that time, even if I met my fellow villagers in other places, I would still speak Mandarin, I would also speak Mandarin when I went back to Dandong on holiday, and I would also speak Mandarin when I called my mother. In short, after more than ten years, I seldom go back to Dandong and never speak Dandong dialect again. Last summer, I tried to speak Dandong dialect again and found that I could only stick to one sentence, and then I couldn't help but change back to Mandarin. Maybe it has something to do with your living environment.