Going up or down a couple of hundred points is normal. In general, you should concentrate on improving in chess, and not on ratings or other numbers.
Looking at your games, my impression is that you just bang out semi-random moves without calculating anything or caring about what happens on the chessboard. This leads to games like this:
Or this:
You should try to pay attention to the game that you play. Try to lose no pieces for nothing and not allow mate-in-1.
I looked at your last 6 loses, and sure enough you've got a common affliction. You're playing fast, probably much faster now than before because you are comfortable with the game and having an easier time making plans and finding decent moves. However all 6 loses you lost with more than half your clock still left.
The late opening/early middle game is the time to play carefully, debate your own moves, and look for tactics. At this level, you'll almost certainly end up a piece, if not several. Until you start losing games by timeout or time scrambles, continue playing slower early on.
You're absolutely right about the opening you've been using. It might be effective with players who aren't knowledgeable about chess, but as you play more higher rated players (who presumably know how to use opening principles and can take advantage of the weaknesses you're creating from the get-go), your usual right-side pawn push opening is not effective. It creates holes in your defense and does nothing to control the center.
Read up on or watch videos about Opening Principles to better understand how to set yourself up in the beginning with a better defense and prepare to attack in a way that coordinates your pieces.
And, as the other posters have suggested, slow down and think longer before making each move.
I’ve gotten super into chess the past few months. I play two games before bed every night and a ton during the day. I was getting super excited about working my way up the ladder, shooting to around 800 at my height. I was destroying all my friends and people in my life thought I was really good. Something must have changed about me, because I then started PLUMMETING. I am now nearing 600 and it’s not getting better. I know I use an antiquated opening, and I could change that to get better, but I was using it before and winning with it. Is it odd that I dropped so much in a matter of a few weeks, or is that just the way it goes? Did I just get unfathomably lucky with my matchups?
Thank you!