Trailering Problems

I am still having problems getting the boat on the trailer smoothly. I am new to inboard driving, but I am getting more familiar with the basics. The main problem is when I approach the trailer, and I bump the reverse to slow down, the boat starts into a left turn. Then when I realize I am drifting out of alignment, and I hit reverse to try again, the boat turns even worse to the left, and I bump the trailer in front of the fender wells. I have tried putting the trailer in deeper, and pulling it out further, but neither seems to help. I am not sure what to try next. Can I power on to the trailer, or do I need to worry about the prop hitting? It makes me really nervous about spinning the prop around the trailer. Any suggestions??