You gotta have thick skin. These things happen to everyone. In your case it is likely that what is happening is that the videos from 365 days ago are now no longer counting towards your watchtime. I'm afraid the only solution is to step it up; focus on making better videos, focus on your SEO Strategy and get your nose back to the grindstone.
yes, its may be because of the season and trends. just focus on your content and quality. Take it as Yt is giving you some time to improve more on your niche. and do the same thing in a better way. and when it's time YT will surely give you views and watch time.