I almost have that same problem...I write the lyrics with a rythm and there is always a faint tune in the back of my head but the lyrics take over and tune fades away. I loose it! I wish I knew how to hold on to the tune but the fact I don't play guitar or piano is probably the main cause.
If your emotions are changing the lyrics everday, maybe you are writing a new song everyday?
Maybe, try looking at it as if you're just writing lyrics and stop trying to make it match your melody. Let the melody sit and one day, month or year there will be an "ah ha" moment when it all comes together.
just my two cents...
All the best,
Lisa