The project has taken too long.
What if you could make it happen by sheer willpower?
Dreams die between idea and execution, so feel free to overcome procrastination and go for it. But not at any price. The result can blind us, leading to shortcuts, compromises, and even unethical moves. Especially if your identity becomes entangled with what you are doing. This often happens with scientists who are passionate about their work.
The problem is that in research, the outcome is uncertain; otherwise, it would not be called research. You can have an educated guess about the destination, but good luck with knowing the path in advance.
Science revolves around positive results, adding insult to the injury of years of work and not publishing in the end. Prototyping can weed out most later-stage failures, but it is no ailment for the sunk-cost fallacy. The more time you invest, the more you will cling to your idea and the less you want to admit that you were wrong. The lesson is to keep a healthy distance between your achievements (or failures) and your identity.
It was no coincidence that Richard W. Hamming called his talk "You and Your Research" and not "You are Your Research".
Think about
Is there something you only want to finish for the sake of finishing it?
Can it be that you are being less careful to get the results you want? Do you think it is the best strategy for the long run? Would you suggest to a friend to do what you plan to do?
Is your mood tied to the ups and downs of the project?