Are you saying that if you put the rudder hard over, with a good amount of reverse thrust, you can't get the bow pointed where you need? On my 27' with outboard, the rudder still wins over the prop walk if I get enough speed and put it hard over. It's usually OK to give the reverse a lot of throttle (enough to make the rudder work), because the forward gear is so much more powerful, so you can 'stop' quickly.
I can't tell from your description, but you need to make sure you're actually going fast enough. Below a certain speed, the rudder isn't going to work well, and prop walk will easily out-do it.
As I still consider myself new to boats, I can understand the general anxiety about this. Here's two hints that helped me:
1) Prop walk is very helpful, because it means you can rotate and solve almost any problem just by alternately forward/reverse. If things go wrong backing, go forward, and then reverse again, etc, and eventually you'll get pointed the way you need to go.
2) As long as you keep your speed at a minimum (the minimum needed for steerage), and you have someone helping you, and your motor is a good, there's not a lot that can go wrong. If something isn't right, use the motor to bring things to a stop, and fend off, if necessary. Obviously having fenders and a boat hook around help; don't use your body.
My suggestion is to avoid the spring line trick, unless you actually need it. I've seen so much silly stuff at docks by people trying to do strange things with "spring lines," when what they really need to be doing is driving the boat like a normal person. As a novice, it's just one more thing to distract you from keeping track of where the boat is going. In particular, trying various tricks to flip lines on or off cleats can easily go wrong as a novice. It's easy to get into a situation where you're freaking out because you can't get your line flipped off a cleat and you don't realize that you're backing into a crusty pylon.
EDIT / WARNING: Any "advice" I have in the area of docking should be considered suspect in light of a terrible "docking" incident that occurred mere hours after this post: First time singlehanding, bad things happen, the sound of breaking fiberglass, etc