You are seriously overthinking it. Just go ahead and fast!
Many times, a virtuous act brings some benefit to the one who does it. For instance, if you are honest in your dealings with other people, they may start to trust and respect you. The fact that you enjoy being trusted and respected does not detract from the moral goodness of honest living.
The key is love. If you perform a good deed with love for God or neighbor, it is very good.