From the choice of subject matter to use of appropriate techniques, there are many factors that contribute to the success of impressionist photographs.
Confidence comes from practice and perseverance but equally it relies on building a sound understanding and experience of fundamentals of impressionist photography.
The ability to create impressionist effects relies on first gaining confidence with techniques and learning the basic knowledge about light, form and colour. The key to success is practice. The more you practice the more familiar you will become with techniques and other aspects of impressionist photography.
Of course, there may well be parts of an image that you are not happy with. So keep in mind what went wrong and try to figure out how to improve that in the future photoshoot. In this way, you learn something from each photograph and so gradually build up knowledge, experience and confidence.
Eventually, these basic skills will become second nature. Then there is more time to think about the qualities that will make your images personal and individual.