Become not interested in my research topic after two years of my PhD? I am supposed to finish my PhD in four years. But after two years of working on a topic, I become not interested in the topic any more. I found out that the topic is boring. What should I do? Should I continue forcing myself to work on the topic, or should I change to something more interesting? I do not want to extend my PhD since my funding only allows me to pursue the degree for four years.

EDIT:
The research topic is quite old, and I think it does not have too much potential in future development. There are already many methods to solve the problem I am working on, and doing a PhD is not using the existing methods, but inventing new ones; this is rather difficult and depressing.
This is a conversation you need to have with your supervisor. If they are any good at supervising, they can help with strategies to either reignite your interest in your topic or salvage what you have done and apply it to something else. If you dislike your topic now, writing it up will be even worse and you may not be able to produce something passable. This is exactly what PhD supervisors are for.