Contemplation exercise; Help needed

I am at the second mental exercise of Step one. The one preceding Vacancy of mind. The exercise where you are supposed to contemplate one train of thought continously.
I have noticed I can do this exercise pretty well. My most recent exercise seemed to fly by in what subjectively felt like 6 minutes. I even turned off my 10 minute timer and went back to continue my meditation a little bit longer simply because I was enjoying contemplating my train of thought so much.

I only seem to have an issue when I run out of things to think about on a topic. This is most apparent when the topic is either contemplating a solution to a problem, or contemplating an idea.
In the case of contemplating a solution to a problem, I am unable to continue once I find a solution. At that point I have an abrupt pause of "Wait what do I think about now?", and unsuccessfully attempt to go over my issue once more.

In case of contemplating an idea, my contemplation ends abruptly once I have defined the idea or concept completely, in total, in the most concise and complete way I feel possible. At that point there is literally nothing to focus on, and once again I unsuccessfully attempt to simply revise the idea once more.

At this point I begin to have strong distractions as I am merely parroting my own thoughts and not truly thinking about anything. It is more a test of memory than of focus at that point, in my opinion.

I would like your advice on this issue.
The mental exercises are my last hurdle before entering step 2. Once I complete this exercise, and complete the next exercise which is vacancy of mind, I can begin step 2.
All other exercises of this step are completed and continue to be maintained.