Like Nikakwa nicely said, lack of visual input doesn't mean that the dream is about to end. In fact, a much common characteristic of "about-to-collapse" dreams is when you start feeling your waking body (and still, that doesn't mean will/has to end!).

The most likely obstacle here is your excitement, which is causing your body to evoke a stress response that wakes you up. Knowing this, you can start to practice mindfulness by being aware of these moments of "anxiety" and calming yourself down: in a dream, a good way to do this is by reality checking with your hands, as looking at them will decrease your visual field and hopefully help you ignoring what's around you.
At the same time, I'd advise you to look into DEILD: it's a great technique to use when you want to continue with a lucid dream. Do notice though that dreams do come to an end when your REM cycle ends, so an alternative explanation for your issue is that you're becoming lucid quite late in the dream. Still, that is something that you will improve naturally with experience!