Jealousy derives from insecurity/the fear of loss. It is often the product of a feeling of not "measuring up" or being "good enough", and is based in fear.
There may be legitimate reasons why your girl may be afraid of losing you to someone else. To better understand, you need to examine if your behaviors would cause this kind of fear in a "reasonable" person, or if your girl's fears actually stem from a lack of her own confidence which were already present or exist. Remember that you can only change your own behaviors, but you cannot address what is internal to someone else. You can and probably should discuss the matter with your girl openly and without accusation in the spirit of understanding the underlying issues better so they can be resolved.
There may be legitimate reasons why your girl may be afraid of losing you to someone else. To better understand, you need to examine if your behaviors would cause this kind of fear in a "reasonable" person, or if your girl's fears actually stem from a lack of her own confidence which were already present or exist. Remember that you can only change your own behaviors, but you cannot address what is internal to someone else. You can and probably should discuss the matter with your girl openly and without accusation in the spirit of understanding the underlying issues better so they can be resolved.