I know you said not to say "not bending it is not an answer," but I'm afraid that it's the most sensible thing - your cut opens up when you bend you knuckle because you're not allowing it to heal. But if you refuse to listen to that, then (I know this is gross) but your saliva heals wounds very quickly. That's why when you get a cut in your mouth, it gets better much sooner than any other cut outside your body. The best thing you can do though is to see a doctor. There may be something preventing it to heal properly, and putting off checking it out may harm you severely later in life. Hope this helps!