Your post helps a lot - ready to give it another shot.
I was more approaching it like a deer 'jumping' over a car. (We all know how well that turns out. lol).
1) grab Uke, and instead of jumping over, it would be more of a 'roll over'.
2) upon landing dont land on the 'side' but have the hand out before you get to the ground.
(To take off shock from the body? Thats where the relax bit comes in as I want to catch myself with my hand and forearm. Same way I got hurt when I slid on the ice this past winter...would have been great to have done some proper ukemi then.)
Koshinage fear factor! My advice would be to make sure you have a piece of nage when you go over (shoulder, obi, lapel - this depends on the attack) and make sure your hand touches the mat first upon landing (tobu ukemi style). Also, I personally think it easier for ukemi when you are doing koshinage at a decent speed. Doing it slow is more difficult than doing it fast. Of course, don't forget the ubiquitous advice "...relaaaaax...":-)