My manuscript has been making the rounds for a year, and still no sale. Do not make the common mistake of spending more energy trying to get published than trying to become a good writer. Perhaps it is time to revise again. Hopefully you have been working on new projects and have them out there as well, once they are polished and ready for submission. You have a much better chance of getting published if you prove yourself to be a career writer, not just a flash in the pan.
Publishers are more likely to invest in an author who promises to be fresh and prolific.
That is what usually happens with time and dedication. You might find that you will first sell your tenth book, then you will go back and revise the previous nine and sell them as well. Though you don't need an agent to submit to many publishers, many others only accept agented material. However, finding a good agent can be as difficult as finding a publisher. Most will not be interested in you until you have a contract.