There are plenty of rules that can be helpful when writing a song, but only if they are viewed as guidelines. Instead of using the rules as tools to build the skeleton of the song, The strongest lyric will share a lot of these common characteristics.
There is no concrete rule when it comes to songwriting, because you can find a great example that breaks every rule. It’s helpful to understand rules and guidelines as road signs, but it doesn’t mean you have to follow exactly where the map tells you to go. You can take a more scenic route and still end up at the finish line. It's often easiest to take the fastest, most direct route, but the path less traveled should always be a viable option.
A lot of times when I learn something new about songwriting, one of the first emotions that rises to the surface is a sense of lost time...a sort of regret that I may have been doing it "wrong" for so long. But as I mentioned earlier, it's important to view songs more as stepping-stones rather than the other shore (a means to an end rather than the end itself).
“There is no right way to paint...so just paint.” (Nashville's Helen King to her art students)
I would echo that by saying that there's no right way to write songs, so just write.