Nothing to beat a sim. I am assuming you can do slow rolls and/or point rolls if you can do harrier rolls with just the elevator. I started out the same way but the roll rate has really got to be up there for the nose not to fall out. Slow down your roll rate and concentrate a lot more on using the rudder - the elevator will come automatically. I gave the same tip to a local flyer and he practiced it on his sim and now he is doing rolling harriers all the time. So the technique does work - hope it works for you too.