I recommend finding a buoy near you in a place with little traffic and waves. If you cannot find one, make one with a fender. Spend an hour pretending it is your slip. Approach it from upwind, down wind, cross wind. Fast and slow. set a second buoy and do figure 8s around them in reverse.
Learn what prop walk is, and practice going from still (near the buoy) to reverse with the wind in various places. See what the boat does between when you put it in reverse, and when you can steer with the rudder.
Learn what rudder wash is, and practice going from still (near the buoy) to forward, using rudder wash. See how far you can turn the boat without moving forward.
Put you bow to the buoy and use prop walk, then rudder wash, then prop walk, then rudder wash to turn the boat 360 degrees. You can turn it in its own length with practice. Do this first with little wind, then in stronger winds once you master it.
Watch lots of youtube videos. Some really good instructors have made some awesome videos and put them online for free:
If you learn the theory, and then practice in a safe, stress free place, you will be confident in now time. In your marina with obstacles and spectators is no place to learn how to maneuver a boat. Too scary!
Note: make sure when practicing to not get your prop too close to the buoy. You don't want to get your prop tankled in the line. If you get to close, just put the engine in neutral and drift until you are a safe distance away.