Silman's Complete Endgame Course. Even folks who don't like Silman's style will grudgingly admit it is pretty good. Yeah, it's a bit intimidating at first glance with over 500 pages but it's designed to be taken a chapter at a time. He first teaches everything a sub 1000 player should know and then gives you a test at the end of the chapter to see if you should move on. Part two is for players 1000 to 1200 and follows the same format. He continues in this way all the way up to master level.
Struggling with End Games
Also recommend Chessable. There is a free Basic Endgames course by TheoryHack. It teaches a lot of the material in Silmans book. I find Chessable's spaced repetition a good way to get the stuff in my head so I use the two together.
Once you've worked with Silman's Complete Endgame Course for a chapter or two, go to Puzzles>Drills here on chess.com and practice what you've read. You can do the drills over and over and over, and it will tell you how well you're doing and show you the best move if you've not played optimally. I think they're terrific.
I've been on a terrible slide. No issue with openings, and I feel like my middle game is decent. I really struggle with the End Game. What resources should I watch/read to help me with this? I'm getting really frustrated with myself at this point.