Mate I retrieve my Stacer 469 easyrider by myself all the time.
I have a long stern rope and hold that in the same hand as the bow rope, to keep the boat straight.
I walk down the trailer and pull the boat up, clip on the strap, keep the bow line tight whilst walking back down the trailer and then just give it a couple of wraps around the winch post. Seems to keep the bow hard up against the V at the back of the trailer, if the stern is in the current you can keep the stern rope in your hand if you want.
Once it is wrapped around the winch post all is pretty much under control.
Once you start winding you can let the rope off and forget about it.
I used to worry about it a bit until I found the method that works for me. I guess the right method for you will depend on your boat, trailer, current, wind etc but once you find one that works you will be sorted.