I've started the long process of turning riffs/melodies into songs, and one thing I'm struggling with is fitting the original melody (as heard in my head) into a playable rhythm. I feel like some melodies gain their strength from the odd timing, and placing them into a playable rhythmic pattern hurts the momentum.

A current example:

wuas1 - this is the original melody which starts heavy off of beat one, and is syncopated with the rhythm; the issue is I cant play the rhythm while singing the melody because of the syncopation, and if i were to play the rhythm more in line with the melody, then the whole thing just drags out.

wuas2 - this is the melody molded into a rhythm which I can play while singing, but I feel some of the energy is lost in the translation, and it feels sadder overall.

How would you approach to this sort of thing?
You just need practice and getting used to the sound of everything together.

You can record the riff (doesn't matter if it's bad quality/guitar pro/etc) and then record your vocals on top. Listen to it a lot e.g. every day. Hearing a version of the music that's set in stone in some way helps hugely.

In the meantime try to sing along to it and play guitar along to it separately - either just sing or just play. Also try to sing it casually in the shower or while driving or something, without the backing. It will probably slowly come together.