// Redirect wp-login.php to /my-account/ for user activation links 
add_action('init', 'prevent_wp_login');

function prevent_wp_login() {
    // WP tracks the current page - global the variable to access it
    global $pagenow;
    // Check if a $_GET['action'] is set, and if so, load it into $action variable
    $action = (isset($_GET['action'])) ? $_GET['action'] : '';
    // Check if we're on the login page, and ensure the action is not 'logout'
    if( $pagenow == 'wp-login.php' && ( ! $action || ( $action && ! in_array($action, array('logout', 'lostpassword', 'rp', 'resetpass'))))) {
        // Redirect to the desired page
        wp_redirect('/my-account/');
        // Stop execution to prevent the page loading for any reason
        exit();
    }
}