I definitely wasn't standing straight up while airborn. I was ollie'ing off the natural roller, which forces me to bring my knees in, although I wasn't motioning as if I were going for a grab.
Not sure I could tell you what you did wrong without actually seeing it, but yes there's a balanced pressure you should have when launching (shouldn't be standing forward or back, but about 50/50 on both legs). And again try to do the grab motions as it'll force you to compress in the air, keeping you stable.
Natural rollers are great to learn on because you can go bigger and bigger with more speed, but at the same time it's easy to abort (just roll over the jump without launching yourself).
The stuff I like to get air off of would almost not get me airborn if I didn't load up and use my own leg strength to launch off the lip. I like this because you always have an abort strategy. Plus you tend to be more balanced in the air if you're the one doing the jumping (versus riding off a launch ramp).
Might not hurt to get a lesson or two on it. I'm all for lessons if you think something's holding you back. I might take a big air lesson this year. I used to hit massive stuff all the time but I'm getting older and the launch ramps start to scare me now! lol