If it’s any consolation, it took me about ten years before I even began mastering reversing into a bay, and not over steering. For me, the trick is to “go in steep”. By that I mean exploit the space you have to get the trailer lined up with the bay as much as you can before your back end has entered the bay. When you “go in shallow” you get the rear into the bay when you’re still at 45 degrees. They’re more difficult to rescue even for experienced drivers.
It’s very difficult to explain. It’s a knack that comes with experience. But one factor which is definitely true is that fewer and smaller movements of the steering wheel is what you should aim for. It’s possible to reverse onto a bay in one go with just a few tiny steering wheel movements. Also, and I still do this, take it slower than the average driver. Don’t rush the manoeuvre, you won’t get any more thanks, pay, or recognition for rushing it.
Another tip is to stop, look at the bay, and imagine pulling away from it. What line would the trailer take? Where would it end up once you’ve pulled off, turned left or right, and straightened up? That’s where you start from, and that’s how you go in steep.