I think being nervous is just going to be a normal part of your trips. We started chartering a little over 2 years ago. First was Oct of 2009 and it was a 47' Catamaran. I was definetely nervous as I had never handled a boat that big before. It took some getting used to and I skipped topping off water one day since I wasn't comfortable with the docking situation. By the end of the week I brought it in and backed it into the dock no problem - just took my time.
2nd Trip was October 2010 and I felt more comfortable going in and got more and more comfortable as the week went on. Docked a number of times without issue - most of them being along side a long dock, not in a slip.
3rd trip I just returned from yesterday
This time I felt very comfortable although still a bit on edge when manuevering in tight quarters, especially with the winds we had. I pulled into a slip a number of times this trip with no issues any time or any major concerns.
So, I think as you get more experience you'll feel better - even if there's a lot of time between trips. I'm still not all that interested in anchoring, although next trip my wife and I decided we'd try to anchor whenever possible just to get more practice.