Being nervous is healthy (I still get nervous after 12,000 hours) but only if it doesn't effect your performance. See the Yerkes Dodson law graph below.
Try to keep yorself in the middle of the graph. If you feel yourself getting too far to the right reduce your workload (ie following the go around you describe consider leaving the circuit for a few minutes, calm yourself down, then try again).
Familiarity will also reduce your stress levels. Take the oppertunity to go along with qualified pilots on their flights if they have a spare seat. Get used to pushing the boundaries of your comfort zone a little (but not too much).
Hope this helps.