I'm stuck at 500, with 10 wins on one day, 10 losses the next, help.

Im 500 in rapid, 300 in blitz and i havent in many months despite trying my best improved at all. i will proceed to win 5 games in a row and loose the next 5. first i play with 80-90% accuracy, then i play with 20-40. this inconsistency of blundering pieces and realizing it the second after i've done it really hurts. i stopped with blitz and only do rapid, but nothing is working. puzzle rush i can get up to 21 streak. and im 1300 something in puzzle score. I genuinely need help. I don't understand what's going on.

/ You castled into this mate. Did you see that mate in one was threatened after Qb8?

How did that happen? A move earlier you were doing well.

What did you see here?  Maybe you saw that your c pawn was hanging.  Did it ever cross your mind that castling towards an open file might be bad?

You are playing 15 minute games but you still had more than 15 minutes left when this move came up.  If you start using more of your time, you may end up seeing more things.

I looked at some of your games. Actually 6, and in none of them did you "complete"  your development.

In the opening both sides are too disorganized to attack, so the best way to play is to bring out your knights and bishops as quickly as possible, castle, and move your queen. When there are no pieces between your rooks on the back rank, your development is "complete." The link at the bottom of this post is an article that explains the opening principles.

Here's are two examples of what completing development looks like:

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https://www.chess.com/article/view/the-principles-of-the-opening

So the opening is a problem, and also you're giving away pieces for no reason.

You have to be really greedy. Don't lose even a single pawn for free. When your opponent attacks something, you're required to defend or retreat. Required. You don't have any other options. Sure GMs will sacrifice and counterattack and play fancy things like this, but right now just focus on not losing anything for free. Not even a single pawn. You must retreat or defend when your opponent has a threat. I know I'm repeating myself but if you give away pieces for free nothing else matters. Tactics, strategy, etc, none of it matters if you gives things away.

So in the opening complete development as quickly as possible, and after that focus on being greedy. Don't give anything away for free.

Do those two things and your rating will improve a lot.

Llama47 is wise.

Re development, have a look at an early Carlsen game at https://www.chess.com/forum/view/general/im-stuck-at-500-with-10-wins-on-one-day-10-losses-the-next-help#comment-66425523.  At move 13, he has all his pieces ready, and the game is won 6 moved later.

The way to think about a development advantage is "I am temporarily up a piece if I move my pieces out".  In the Carlsen game, he was two rooks up "temporarily".

The second point from llamas47 is about unprotected pieces.  Try to take an inventory during your opponent's time of which pieces and pawns (yours and the opponent's) are undefended.

Always check your intended move is no blunder before you play it.
Whenever you lose a game, stop playing and analyse it first to learn from your mistakes.

i try. it just seems like nothing stick. i actually spend a good time analyzing. it just doesnt stick. mostly because i dont understand what im analysing.

Play 30+0 or longer exclusively. Spend at least 10-20 seconds checking that you aren't hanging anything. It helps.