Read More link troubles

This “read more” function is something I seem to struggle with on every site I do and finally want some clarification.
In my post I have put a line of text, then <– more –> and then another line of text.
In the blog page template, I have the_content(); and in my functions.php I have

function my_more_link($more_link, $more_link_text) {
	return str_replace($more_link_text, 'Keep reading this post', $more_link);
}
add_filter('the_content_more_link', 'my_more_link', 10, 2);

In theory, this function should insert “Keep reading this post” at the point of my <– more –> right? Well it doesn’t, I still just get the full post.

Any help please?