Its pretty simple, people don't appreciate how powerful the mind is. Your mind knows it doubts it. Have you ever heard of affirmations? They are verbal things you say, like "I can do a very good job on my presentation today". They are often used for goals. they are scientifically proven to help with achieving what you want.

Essentially what your doing is the opposite. You know the power of your mind, otherwise you would not be on this website. You know if you believe you can have a lucid dream, you will, but at the same time you know if you think you cant, you wont. Achieving a lucid dream is like control, you know if you believe 100% you can do whatever you want, you can, but if the little bit of you thinks, it wont happen if i don't think it will, you make that impossible.

So you need to look at it the other way, literally anything is possible for a dream, if you think it can happen, it is that simple, same with getting a lucid dream.

Beyond the "train of thought" i don't know if you have tried WBTB, but they are one of the most consistent methods you can use, go to the tutorials and read about it.