Julianne, I finished two portrait commissions for Christmas gifts and haven't been back in the studio since then. I have a commission pending but don't have the photos yet, and I'm glad to be taking a break. At other times I go right from one to another but if you're finding that the pressure of work is getting to you, you should definitely allow yourself some downtime. This is a great profession but to do your best it should be fun and exciting to embark on that next painting. Go to a museum or look at your library of art books or do something else to refresh your spirit!
Another thought: if you're that busy, it may be time to think about raising your prices!