I’m new to using Audacity and have tried the noise reduction feature to no avail. I’ve also gone into play with a few other things, but nothing seems to get rid of the horrendous speaker feedback drowning out nearly an entire song on an MP3 from a concert I went to. Any suggestions? Better yet, would anyone be open to taking a look at it? I’ve got 3 others equally as bad and I feel kind of dumb for not being able to fix them 
I feel kind of dumb for not being able to fix them
You shouldn’t feel dumb. Chances are good nobody can fix them. Any posting with the words “Clean Up” or “Concert” is usually the kiss of death. Recording a concert is difficult for the big kids. It takes special microphones, attenuator networks, blast filters and monitoring systems to pull off. It’s not unusual for the recordist to just get a track from the house rather than trying to make it themselves.
It’s not at all easy.
You can post ten second mono or five second stereo clip of the damage here on the forum, or point us if you have a file posting service somewhere.
https://forum.audacityteam.org/t/how-to-post-an-audio-sample/29851/1
Koz
A good example is to take a good look at the Royal Albert Hall in London where the BBC are currently televising some of the Proms concerts. If you will look carefuly you will see that they have many, many, microphones about the hall - and not all the same. Then look up the ceiling and see the little white “flying saucer” jobbies. They were retro-added at a fair cost to improve the sound for recording. It ain’t easy as Koz points out …
See this image from Wikipedia: Royal Albert Hall - Wikipedia
WC
An interesting trivia fact: the acoustic treatment of the Sydney Opera House cost more than the original estimate for the complete building.