Not motivated for the second part of thesis
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.