Keep in mind that whipping cream or egg whites by hand may take longer than you expect.
That said, there is a proper way to whisk egg whites, and it is quite likely that taking breaks due to your fatigue is interfering.
See this site for detailed instructions. In particular, see below for an excerpt on one possible problem (although there are numerous things that can potentially go wrong).
We will split the whisking of egg whites into two parts. The first covers the disintegration of the mucous mass up to the point when it becomes a grayish, roundish mass. This part requires mixing with an easy and rhythmic effort. The second part covers the conversion of the round, gray mass into a smooth, light, firm, stunningly white batter. This second part, unlike the first, requires a very vigorous and accelerated effort.
During both these periods, the whisking, even though different in strength and speed, must not be interrupted. Under no circumstances should you stop once you have begun to whisk. Stopping causes the whites to disintegreate. Instead of rising and firming up more and more, their mass will remain semi-solid and will take on a blotchy, grainy look.