I worked with The Beatles in
Germany during 61 62 63

They got better and better every season.

A lot of those hit songs were
formed during that time, but as they got better writers, they re wrote them into hits.
In the early days the songs were too long they learned that to keep the listener interested, they adopted what they named The Four Bar rule, basically speeding the flow and slowing the flow,  it’s a bit deeper than that , but remember the attention span of your listener,  you can keep that by variation of the chord count per measure.  example one chord for the first four bars then maybe two chords per bar for say two bars followed by one chord lasting over four measures or bars, avoiding senseless repetition.

Every song must have repetition but too much spoils the end result , the cleverness is knowing how much. 

Listen to a playback of your song, and improve it , that’s what Top Writers do all the time.  Dont rush out and pay good money for a demo of something that is not ready.

Yesterday is a fine example of a Great Song

Yesterday 3syl.
.All my troubles seemed so far away. 9 syl.
Now it looks as though they’re here to stay. 9 syl.
I believe in Yesterday 7 syl

Yes today the rhymes look a bit cliché, but remember it was written in the early sixties.
Etceteras.