#1
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
WUAS2

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?
#2
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.