So there's no real music theory answer about how to write good melody lines as it all depends on your subject matter. Typically for pop songs the rhymes are pretty clear and obvious. Taylor Swift for example typically uses either ABAB or AABA for her rhyming pattern. Pop songs are designed to be catchy so if you use a complicated rhyme scheme it will be harder for people to be able to sing it without listening to it. Choruses for pop songs are almost always either constantly rhyming or ABAB. For pop songs I would keep the pre-chorus to around 2 lines and make sure they rhyme. Typically, you rhyme clauses and not just full sentences so don't worry about trying to make only the ends of the sentence rhyme.
Melodically, I would consider the mood of the piece primarily. Also the main message you want to put out. That will help with the chord progression. I would say 99% of pop songs are 4-chord songs (usually I V VI IV or in chord form: G D Em C ) because that is the simplest chord progression to write over. My advice would be to write out your message first and maybe try to rhyme that. Then start singing a melody line until you find one you like and build off of that. It takes a lot of trial and error. When I write songs I sometimes go through 20 different melodies and end up picking one completely different. The most important thing for a pop song is catchiness so if you come up with a melody that gets stuck in your head go with that one. You seem to have written songs as well so you understand catchiness. The melody that gets stuck in your head is referred to as a hook so make sure you have a good hook that sticks in people's minds. That's really how pop stars write their songs. They come in with a really catchy hook and build off of that. So start fooling around on an instrument or singing until you come up with something that sounds good and is catchy.