why does my discharge change so much (I’m 12 8 months, no first period yet) and why does pasty texture discharge only last like 8-10h then leaves

Your discharge changes because your hormones are changing as your body grows, which is totally normal.

Before your first period, your ovaries are slowly waking up and sending out different amounts of hormones, especially estrogen. As hormone levels go up and down, discharge can change in texture, thickness and amount. Normal discharge can be thin and stretchy or thick and pasty, and it can switch between them, so a pasty patch that comes and goes is part of that. Your body makes discharge to keep things clean and healthy down there, so it is just doing its job on a moving schedule.

Clear, white or off-white discharge is normal in puberty and can start some months before a first period, and it can look a bit yellow once it dries on your underwear. Tell a grown-up if it comes out a different colour (like deep yellow, green or grey), smells strong or bad, turns lumpy like cottage cheese, foamy or like pus, suddenly gets much heavier, or comes with itching, soreness, pain when peeing or tummy pain.

Wash the outside gently once a day, never inside, with no soap, shower gel, perfumed products, bubble bath or douches (a grown-up can help pick something gentle to wash with), and wear cotton underwear and nothing tight. If you get itching or soreness that doesn't settle, or it comes with a change in discharge, a smell or pain when peeing, tell a grown-up, who can get a doctor to check.

Since discharge is one of the big signs your body is getting ready for a cycle, you can check out this quiz if you want to see where you are: how close is my first period?