Can you get a picture of your slip setup?
It would help us visualize what is going on.
As you get better at this you will find that you will go through a couple transitions.
At first you will only be comfortable with crew.
After you get better at doing it yourself you will find that having crew makes you nervous and you prefer doing it yourself.
The place you put the spring line can be adjusted so it balances the boat.
I'm not sure it is highly recommend but on boats without a center cleat I have attached it to the rail if it is one of those rails with slots or to the base of a stanchion, usually amid ships somewhere works well.
If it is not placed properly the stern can indeed drift off. Placed properly with the wheel turned toward the dock it should work great.
This assumes your dock extends long enough which is why we could use pictures.
What boat do you have, length, width, engine etc?
Sounds like you have a challenging location.