Forget you're a guitar player when you pick up the mandolin. Never mind that mirror image stuff (unless you're like jonm, I never realized that). The fingerboard logic is that of a fiddle. The right hand style is very different from either guitar or fiddle--you have to develop a mandolin right hand,learn chop, tremolo, no fingerpicking...

Get a book of fiddle tunes and try them. Almost any fiddle tune is easier to play on the mandolin than the fiddle, until you get to a certain (very fast) speed where frets and pickstrokes become obstructions. Learn the tunes single-note, then add double-stops, throw in some tremolo here and there...

Just do it, you won't regret it, they are complementary sets of abilities. And if you go to sessions, you're a more popular character if you can sense which one you should play given the circumstances of a particular set...

W-O