Hi Frances

Welcome to the club. I think most of us have had a few falls while trying to get a grip on clipless pedals. I think its a practice sort of thing maybe you could find a safe flat place to ride around and just clip in and out until you feel a little better about it.

The type of pedals you choose will also determine how easy it is to clip in and out. Some of the wonderful ladies on this forum do Tri's so they may be able to help with pedal selection and tips for changeovers in a Tri.
Put a post on the Tri section Im sure you will get loads of good advice from them.

Happy riding.