Single or twins?
If it's a single, you are experiencing the effects of prop torque when you reverse. Depending on whether it's right or left rotation, it'll walk the stern a bit sideways.
Solution: take the boat to an area where there is little wind or current and reverse, watching what it does. It's much easier to analyze what's going on when your not inches away from a piling. Once you know what your boat does while backing down, you can account for it in your maneuvering.
If it's a twin rig, that is a whole other thing. Once again, go to a deserted area where you can play with the boat. You'll see how you can spin the boat and maneuver it with just the engines. I have found that trying to turn the wheel while utilizing the twins is counter-productive. One action undoes the other. I have had the best luck with the helm centered and the engines trimmed up a little so the propwash doesn't hit the transom, limiting thrust and maneuverability.
The nuances of close-quarter twin engine handling is too involved to explain here. One guy on here had a good point..Look at your Yamaha throttles. The handles point inward. If you want the stern to go to port, put the starboard engine in reverse, as the stbd. handle points to port(or left.)
One more thing..There will always be "those days." Wind, current, and honest mistakes will screw up your landing. Blow if off and try again. Eventually, you'll get it, and you'll think intuitively, instead of being stressed. We've all been there..Good luck.