forget about scales and theory for now - just do the melody.
do it strong and clear. try the smallest variations off it.
work on slow tunes. that's what jerry did for Positively Fourth street, Dixie, etc.
don't force it, maybe add a passing note or two to the melody, take your time, no hurry. eventually you'll see passing notes and scales that fit in, but don't worry about it.
good exercise? do a couple of very very simple bb king riffs (I have that on one of my youtube vids, I think). work on your tone and phrasing. sounding good has nothing to do with how many notes you play - it's how you tell your story.
good luck, bud.