High Sierra 10.13.6 – Security update 2019-003 failing install

My 15" MacBook Pro (Retina, Mid 2012) is running slowly since I attempted but failed to install Security Update 2019-003 High Sierra. After researching the problem, I discovered a number of reports indicating the update has failed other users and quickly decided I will wait for Apple to release an "updated" update. However, since attempting the failed install, my machine is running so slowly I can hardly use it. It's as if it's trying to move "under water". Something as simple as moving a document to a different folder stalls the machine as does opening a Web site. I've tried rebooting to no avail and testing the speed on my IP. I know enough about my Mac to realize that I know very little and am terrified to start opening folders and deleting things without advice from someone who knows what needs to be done. Now that I've revealed that I am a complete computer dodo, I hope not to embarrass myself further. History: I received an automatic update from the App Store that a restart was needed for Security Update 2019-003. However, when I clicked restart, my MacBook Pro changed to a black screen with only the cursor showing. The next morning nothing had happened and the only option I had was to do a hard shutdown and restart. When I saw the upgrade had not been installed, I continued after making room on my disk. After several failed attempts, I found information that the update had failed other users with the same circumstances that I encountered. My problem is that I don't know how to clear out the portion of the upgrade that is there and ready to install, that I believe is causing my machine to run so slowly. I've searched many times and ways but can find nothing on how to remove what was downloaded and pending installation after restart. Does anyone know where I can locate this on my Mac and how I can rid it from my life? The upgrade is obviously flawed and I just want it trashed from my machine. Any help is appreciated. Sincerely, Kathryn