Very Bad jumping physics in Godot

player.gd

extends KinematicBody2D

var screen_size
var velocity = Vector2.ZERO

func _ready():
	screen_size = get_viewport_rect().size
	$CollisionShape2D.position.bounce = 0

func canjump(): # A temporary function. Will return more useful data later
	return true

export var fall_gravity_scale := 150.0
export var low_jump_gravity_scale := 100.0
export var jump_power := 10.0
var jump_released = false
var earth_gravity = 9.807 # m/s^2
export var gravity_scale := 10.0
export var speed = 400

func _physics_process(delta):
	velocity += Vector2.DOWN * earth_gravity * gravity_scale * delta

	if velocity.y > 0:
		velocity += Vector2.DOWN * earth_gravity * fall_gravity_scale * delta 

	if velocity.y < 0 && jump_released:
		velocity += Vector2.DOWN * earth_gravity * low_jump_gravity_scale * delta

	if canjump():
		if Input.is_action_pressed("jump"): 
			velocity = Vector2.UP * jump_power
			jump_released = false

	if velocity.length() > 0:
		velocity = velocity.normalized() * speed

	position += velocity * delta
#	position.y = clamp(position.y, 16, screen_size.y - 16)



	velocity = move_and_slide(velocity, Vector2.UP)



func _process(delta):
	var dir = 0
	if Input.is_action_pressed("reload"):
		get_tree().change_scene("res://assets/scenes/load.tscn")

	if Input.is_action_pressed("move_right"):
		velocity.x += 10
	if Input.is_action_pressed("move_left"):
		velocity.x -= 10

	if Input.is_action_just_pressed("LMB"):
		var mouse_pos = get_global_mouse_position()
		position.x = mouse_pos.x
		position.y = mouse_pos.y