About the letter, I'd start with introduction.
Tell them about you....( My name is Darla ...
Summarize your dental background and what you like about dentistry.( ie; I have been in the dental field over 20 years dental assisting and____. I especially enjoy helping the patients feel comfortable and at ease. I also enjoy doing lab work and deliveries. )
Tell how they would benefit from hiring you .(ie; I have thoroughly enjoyed being in the dental field exclusively over the years. There is much I know and could offer , though, I am very eager to learn new things. I feel that as a dental professional with over 20 years of experience that I would be a asset to your practice.
Well, something like that . Tweak and add . and GO FOR IT.