Mine rolls straight off also, so this is how I launch solo - before approaching the ramp, I run a line from the midships cleat forward to the front of the trailer, and then back to the same midships cleat where I tie it off. I make sure it has a little slack so the boat can settle back onto it. Then when the trailer is in the water I unhitch everything and let the boat settle back onto the line I tied off. I then get in, start it up, drive forward to take the strain off the line, undo it and pull it into the boat, and then reverse back off the trailer. Works surprisingly well. For reference I've got a Senator RH620 with a 140 outboard.

Haha. Good old ermo boat ramp