Many years ago I worked for a contest, The American Song Festival, located in Hollywood CA, and was in contact with many "publishers," used as judges in the lower levels. (out-of-work publishers called themselves "independent producers") Also sat in and helped tape a songwriting course at UCLA taught by Al Kasha (He wrote "there's got to be a morning after, if we can hold on through the night"). The publishers, many who "fell" into the music business used to say, "If it took longer than 20 minutes to write, it's no good". Al Kasha approached songwriting as story telling. By the end of verse 1 you should know who the people are and what the conflict is. (i.e. Don't sing love songs, you'll wake my mother she's sleepin' here right by my side. And in her hand a silver dagger,- in other words - we love each other but Momma's gonna stab you) Verse 2 you come in for a detail of the picture - a close up - something that intensifies the conflict. Then to the chorus/bridge - the "hook" -Philisophical statement that explains the song. I'd tell these publishers that I'd written a song. All they'd ask is, "What's the hook (title)". Verse 3, resolve the conflict. (The resolution can be that the conflict is going to continue)
If you can put a girl's name or a state name in the song - so much the better. The judges at the Song Festival complained all the time about listening to boxes of 100 cassettes and being unable to find a positive love song. Take that song where you said "I hate you" and change every "hate" to "love." "I'm so sad, life is terrible, boo-hoo, whine-whine" songs might work for you but other people mostly want something positive. Keep your language simple. If the listener has to stop and try to figure out the meaning of the line he just heard, he misses the next line and the song is lost on him. Also exagerate. You don't say, "I'm gonna' love you I think, for a while, maybe". No! - say "I'm gonna' love you 'till the end of time!!"
As to melody - you should be able to sing the song without an instrument backing it up. If you take away the instrument and all you have for a melody is changing back and forth between two notes - you don't have a melody. Lucky Carl, a publisher, told me that he knew a songwriter who came to him one day and said he'd written a song. Lucky told him to whistle the melody. The guy whistled "That's Amore" Lucky told him, "It's a hit" Melody should mirror the words. If you say you feel "up" the melody should go up. If you sing about the moon, the melody should sound like the moon.
After all that - if you look around you'll find that many hits do not follow any of the things I just said. ?????