Understand the meter of the language you're writing in is critical to making an effective melody, and it seems like you have that down.
Before working on the melody, it helps a lot to have effective lyrics, and by "effective", I mean lyrics that will work melodically and that almost suggest a melody all on their own.
The hallmarks of effective lyrics are easy to recognize once you understand how lyrics work with music. Music is all about repetition and change. A sequence of random sounds is generally not considered music, so repetition of a sound or pattern of sounds is an important part of establishing that the sounds are musical. But endlessly repeated sounds are boring, so usually in music we hear a figure repeated a few times and then a different figure is repeated. Each figure may or may not be related to the last one in some specific way.
Analogously, repetition and variation are also important in lyrics. That doesn't mean you have to repeat the same words, but you have to repeat something. A very popular aspect to repeat is the meter. So your lyrics should have repetition and variation of meter. If you're struggling with repeating lyrical elements, an easy out is to literally repeat the same words two or more times. This may look boring on the page, but it will help inspire a melody that repeats the same rhythm with different notes.
Once you have a grasp of the meter of your lyrics, you can start writing a melody by merely composing a rhythm that matches the meter. Even just one repeated note sung with an interesting rhythm can form a melody. Listen to the verses of "Welcome To New York" by Taylor Swift (from 1989) to hear repeated lyrics with a repeated rhythm and very simple pitch variations. It might not be the kind of melody you want to write, but it's a simple way to get started.
Once you have a start on the rhythm, you'll probably want to start varying the notes. You can compose very complicated melodies starting from a very short, simple motif. Writing that first motif is probably the hardest part, but you can take inspiration from your lyrics. A good way to write the first motif is to focus on a single line of lyrics.
If the first line of the lyrics is a question, then maybe you want to end the first line of the melody on a high note. A declarative statement may end on a lower note or have a sort of cadence at the end. If you want an inspirational sound, then a rising melody can help. Alternating between two notes can lend a more negative feeling. One thing I think about a lot when writing melodies are what I call "bugle notes" - the notes that can be played on a bugle. These are basically the notes of a major triad in inversion. Basing a melody on bugle notes can help make it catchy and poppy, while avoiding bugle notes will help avoid sounding too poppy or happy. One classic example of literal bugle notes is "Taps", the bugle music used for the end of the day and funerals in the US military. A happier example is "Reveille", which is used at the start of the day. Studying those can give you some real insight into how simple melodies can be very effective.
If you already have chords, then you probably want to start and end a line on chord tones. When selecting chord tones to be part of the melody, it helps to experiment with how each note interacts with the chord, meaning when the melody note is the root of the chord, it will have a different sound from when it's the third of the chord, and likewise with the fifth. I find the melody note being the root of the chord is the most "boring" sound of the three, and generally I only do that when the chord itself is interesting in a harmonic way.
Avoid a motif (the short melody for the first line of lyrics) that seems to resolve at the end. Motifs that end with more tension are best. Most melodies maintain some level of tension for the entire song until the very end. This is also good advice for chord progressions. Hans Zimmer describes melodies as being either "questions" or "answers". Most music is almost entirely "questions" with very few "answers".
Once you have a motif, things should get easier. You can repeat the exact same motif several times before it gets boring. You can also repeat the same motif with different chords under it to make it more interesting (see Delta Blues, where each lyric motif is sung over the I chord and then often repeated over the IV chord).
You can also change the motif in simple ways to continue the melody. My favorite change is sequence, which means to shift the whole motif slightly up or down in pitch. Another common motif modification is to reverse it ("retrograde"). You might hear a melody line go up for the first line of lyrics, and then go back down for the second line. You can invert the intervals or make one of several other changes. You can also keep the same pitches and change the rhythm, or vice versa.
Another common element is a countermelody, which is a second motif that plays against the first one. If your lyrical lines alternate back and forth in meter or rhyme, a countermelody for the second line might be very effective. Then you could alternate between the two motifs just as the lines and rhymes alternate. Countermelodies are great for harmony vocals as well.
So to break it down:
- Use repetition
- Start simply - come up with a rhythm on one note at first
- Focus on writing just a short melodic motif based on the feel of the first lyrics
- If you have chords, use them for melodic inspiration
- Work on writing musical "questions" with very few "answers"
- Repeat and/or vary the first motif
- Develop other motifs the same way, either for countermelodies or for the chorus, bridge, etc.