I am asking for a hint about the following issue. I have this particular (let's call it weakness) that I have to be motivated to do research. I have done a large portion of my thesis in PhD, but not for all cases. To put it in an example, assume that you are trying to prove a theorem in mathematics, which has two cases. You and your advisor are both convinced that in principle should not be any difference for the proofs i.e. if you proved the first case, you expect that the second case also holds. However, my advisor really wants to see the result directly for the second case as well. The issue lies exactly here: I am not convinced (motivated, etc.) to study the second case which is just more messy but she insists on this. In this case, I am asking for your suggestions: how would you motivate yourself to repeat the same process for the second case? I should emphasize that the problem itself is very interesting for me, and the issue is not this. The problem is that I am convinced that the second case is just a boring repetition of my first approach. I'd be so much thankful for your suggestions!
Sorry, but there are parts of math that are just messy. Worse, there are subtle parts that can throw you off. If you don't follow her advice you may be stepping into a mine-field.
But, it also sounds a bit like burn-out, and that you need a break. How much time can you take off without thinking much about the problem and come back to it fresh. It may still be a slog, but depending on intuition only gets you to the point where you seek the proof that validates it. Insight and intuition are good (essential), but the proof is in the proof.
Relax for a while (week, month?) and then get cracking.
But also guard against being sloppy in your thinking. If the second case really is a bit more subtle and difficult then (a) you might skip over the real issue and (b) that part might be the essence of a better paper/thesis/whatever.
I actually struggle a lot with the motivation. For every task I'd like to do, there are three that I find boring. I came to understand though, the boring stuff is also needed. Sometimes, it even helps to understand the problem or in case of writing the reports, it gives you a look back at what you've achieved.
One of the motivation techniques is to divide the task into easily accessible milestones with, if possible, achievable deadlines. Write them down, keep them in the visible place and strike them out when done. It's really satisfying. If that's not enough, you can also plan some rewards when you achieve the goal.
In your case, you could also focus on some side project for a time being, so you come back to the task you are now bored with with a fresh mind. It's good, however, to set a date by which you start to work on it again.