Your self-image has a major affect on your character and personality. The way you view yourself becomes your reality, for better or for worse. This concept is so powerful that it will make a difference in all that you do and don't do your entire life.
View yourself as a person who is enthusiastic. Even if you aren't always enthusiastic, as long as you are enthusiastic some times or have been enthusiastic in the past, it means that you are a person who has experienced enthusiasm. And since you know you have already been enthusiastic, it means that you can view yourself as someone who will be enthusiastic more times in the future.
Every time you are enthusiastic, you strengthen your view of yourself as being someone who is enthusiastic. So the very next time you experience enthusiasm about something positive, you can say to yourself, "I'm enthusiastic now, and I am an enthusiastic person." As you identify with your positive qualities, you make them an integral part of who you are.