Tannin,
Keep practicing and you will get better with the creaser. Corners are difficult at first but you will get better at it. On a rounded corner, I will move the creaser and the leather at the same time. You will eventually get a feel for it.
Also, it really isn't necessary to use the creaser hot on veg tan leather. I use mine cold all the time, starting with a light line on the leather, dry. After I have a line set, I wet the leather, let it come back close to its original colour then do it again with lots of pressure.
Just curious, what type of screw crease are you using?
Keep practicing