Child style loads before all parent styles

What I see here is you are not using dependency parameter in your child theme to load the css.

wp_enqueue_style() accepts 5 arguments.
The 3rd one is dependency. This dependency allows your css file to load after that.

So if you will add the parent stylesheet handle in child dependency then this will load after your parent css file.

For more information on wp_enqueue_style() please go through https://developer.wordpress.org/reference/functions/wp_enqueue_style/

So your code will be

// Child Theme Stylesheets
function my_theme_enqueue_styles() {
    wp_enqueue_style('stylesheet',get_template_directory_uri().'/style.css' ) // parent theme style.css
    wp_enqueue_style( 'child-style', get_stylesheet_directory_uri() . '/style.css', 'stylesheet' ); // child theme style.css
}
add_action( 'wp_enqueue_scripts', 'my_theme_enqueue_styles' );

Here the 3rd argument is the handle of your parent stylesheet.
Hope this helps you out.

Note : You can use get_stylesheet_uri() function to load your child theme’s style.css file.

Update:
Please go through this link. Loading a child-theme’s style.css after the parent’s

Thanks