How do I keep believing in my research after years of failure?

You are walking a narrow line, holding on to hope that the failures mean something, while refusing to lie to yourself about the long odds. That is hard, and most people fall off one side or the other, into blind faith or into giving up. Staying honest and hopeful at once is a real discipline. Here is what can hold both: the meaning of your work does not depend on you personally reaching the cure. Every dead end you map truthfully is a path the next person will not have to waste years on. That is real service, even when your own experiment fails. The failure is not wasted if it tells the truth. So separate the two questions you keep tangling. One is honest: what do the data actually say? Answer it cold, without hope's thumb on the scale. The other is about meaning: is this work worth doing? That one can stay yes even when today's answer is no. Keep them apart, and you can face the odds without losing heart.