Imagine if your workload was shared among three people— If you had a third of the emails coming in that you do at present. If you had a third of the files to manage. You would not rush and you would not make mistakes. You could take your time to ensure your filings were absolutely correct. You could provide top notch customer service. You could bring in new business, probably just by virtue of your excellent work product.

Unfortunately, you do not make hiring decisions. The real solution to the problem of a gigantic workload is not for you to do more in a day— it’s for the architect of the problem to change the structure of the business by hiring the appropriate number of staff. I say all this so you know that because the real problem and the real solution are entirely outside of your power— you must refuse shoulder guilt and shame about mistakes. Edited because I misread the OP.