Why did I suddenly get bad at chess?

I'm nearly 70. Blind in one eye and only abour 25% vision in the other.

I stopped playing actively when it became difficult to look at a chessboard for more than a few minutes at a time.

=> Okay that's fair enough, it's just that you're playing on an account that gives 0 proof you've ever been good at chess

Check a few of my games, perhaps?

Here's one from a rated country vs country team match here on chess.com:

A Heroic Defense in the Sicilian Najdorf - Kids, don't try this at home! - Chess Forums - Chess.com

Others available on request.

A Rapid rating of 900 Glicko is way above average. Do all the intermediate drills and lessons first.

Oh cool, okay. Looked you up and didn't realise you were insanely active. Makes sense why you have so many profile views.

Awesome, again, respect to you, you're what we call an "OG" lol haha if u know what that means. No, seriously though I can tell that you're mature and knowledgeable about Chess and you have dropped some of that knowledge right here on the site.
I think, and I say this almost jokingly speaking that maturity starts in Chess with long term strategy, you first start playing Chess, you're just pushing pieces, your attacks are just there because............lol, after a while you know you start to formulate....I don't know, I think you get the idea, it took a while for it to click for me but I'm far from my definition of "mature" lol. Well I'm trying to learn to plan, I still have alot of Chess misconceptions" .........."yeah but what if he goes there then what?" lol........
I am and always will be a student of the game!

im experiencing this, this month im new to playing online im almost at 600 elo when suddenly i started to blunder obvious blunders and started making questionable moves and now im 300+ elo i dont know what to do pls help chess is my life and is my favorite hobby

The key is practice, if you say this is your hobby then you ought to want to practice alot. In other words, play at least a few Chess games a day, preferably slightly longer games where you can think some about your moves. 
As far as blunders are concerned, it depends on what types of blunders that you're making, often at times players leave pieces en prise, as in, just to be taken for free, in that case, I'd recommend you make sure all of your pieces are safe or well that is to say protected during the game. I'd focus on that first if I were you. In Chess there is alot to take in during the game, and even though I've been playing for years I still get overwhelmed at times, so you should focus on improving and learning about one thing at a time.

Your rating will drift around by sheer chance, even if your intrinsic playing strength stays constant. I figure, in a hundred games, you rating is likely to range over plus or minus 75 points of your intrinsic playing strength. If you play a thousand games, your likely range is close to plus-or-minus 100 points.

I figure that based on some computer simulations I did with a simple Python program, which you can download at https://strnbrg59.com/elo_random_walk.py

Maybe this knowledge will be therapeutic for you (it was for me!) in that you can stop stressing about movements in your rating, knowing that even a hundred points down doesn't necessarily mean you're losing your chess mojo.

i do play everyday both at online and in real life thats why im confused why am i suddenly getting worst and worst each time i play, and when im watching someone else play chess i know what's the best move they can do and i can predict theyre moves really well even though theyre elo is higher than mine but once im infront of the board im suddenly blundering making mistakes etc im really confused to whats happening to me, thank you though, i just really want to rant this out my closest friends doesnt understand me (srry for the bad grammar )

Alright, I understand the frustration, I'm going to see if I can shoot you a message real quick.

I also need help I'm failing at passing 200 ELO and I'm trying but I blunder my knight like every game in like 10 moves or less accidentally.

The joy of winning a 30 min game by checkmate, is so much higher than winning a Bullet game on time. Using your brain fully (for online games) and calculating slowly all the way to victory.

Conclusion of this dissertation: Play at least 30 min games for a couple of years!

We're ALL bad at chess.

Some of us are just worse than others.

I literally have no idea what is going on with me or my game. I was on my way to an 800 Elo rating. I noticed I had won 10+ games back to back by checkmate, so the next day I played distracted. Most times I win, but this time I lost and it ruined my streak, and ever since then, I have been unable to play smart chess games. I am currently at 400–500, so you can imagine the drop from 800–500. I literally don't know what to do. I deleted the game out of anger. I even take reviews seriously, but I am getting dogged on by people on 500. It's annoying, and idk, it's too much for me. I think I might quit chess. ohh also I have another account so this is my burner account the losses happened on my main account and it hurts .

This graph is made from dunning-kruger effect graph.

Formula: (Competence - Confidence) * Competence

Crazyhouse is a chess variant where any of your opponents pieces you capture can be put anywhere on the board