Hey as a relatively newbie-beginner, myself included this is where learning to save a copy comes into play. There are no tutorials or videos only something I have learned the hard way. I have wasted many hours of diligent modeling hours only to realize that FILE > NEW was what I had to do. The only good is that it is truly practice for you next attempt. Your issue is just one of many others that will arise but can be avoided. This simple step allows you to go back passed the default number of undo`s available, then gives you a fresh amount to utilize before the next saved copy. Make this part of every model you do…regardless.
You can change the face colors from the Styles options, select EDIT. So the backside face is much darker or total different. Alerting you sooner then later.
Why SU will apply or it will show a face reversed I cant explain, I have had situations were the results of a applied tool or option. Has reversed it from the others on the same plane, I am still learning. So this will help you to realize it before you spend another hour adding to the file. Then hit that point of no return, WISHING YOU HAD SAVED A COPY. Hope it helps and it even has a key board short cut you may need it so much…Peace…
