On the keeping of tunes in the mind, I, too, fall into the trap of not being able to remember tunes as I write 'em unless there are words to help me.
If I'm just noodling on guitar, I can come up with tunes, but a process also starts as soon as I've played something nice. The process is this: First time around, "Hey, that's nice." Second time around, "Hmmm... that sounds a little like something I heard from someone else (whether this is true or not)." Third time around, the tune has transmogrified into the existing tune that the new tune reminded me of. And there is no fourth time around, 'cause now I know what I'm playing ("Girl of the North Country," or "Itsy Bitsy Spider," or "Silhouettes") and the new tune is long gone. In other words, my picker's brain takes over and tries to identify the tune, usually resulting in finding a tune that reminds me of the new tune and then latching onto that one.
It's only when I have words to hold the notes in postition that it works. And that's one reason why I write lyrics first far more often than I write tunes first. I typically have more lyrics than finished songs.