如何使用HTACCESS将特定.html替换为尾部斜杠

I have this command in .htaccess

RewriteEngine on
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}\.html -f
RewriteRule ^(.*)$ $1.html

And this command remove the .html at the end this looks ok but how can I add at the end? Because if I try this code:

RewriteEngine on
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}\.html -f
RewriteRule ^(.*)$ $1.html

RewriteCond %{REQUEST_URI}  !\.(php|html?|jpg|gif)$
RewriteRule ^(.*)([^/])$ http://%{HTTP_HOST}/$1$2/ [L,R=301]

The .html is there again. I want to replace the .html into a trailing slash. And also I want to do this in 1 HTML file only. So my URL is like this for now.

http://mysamplesite.com/project.html/

You can use it like this:

RewriteEngine on

RewriteCond %{THE_REQUEST} \s/+project\.html\s [NC]
RewriteRule ^ http://%{HTTP_HOST}/%{REQUEST_URI}/ [NC,L,R=301,NE]

RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}\.html -f
RewriteRule ^(.+?)/?$ $1.html [L]

Try below rule I am assuming project is the file on which you want to apply trailing slash.

RewriteEngine On
RewriteRule ^project$ project/ [R=301,L]
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}\.html -f
RewriteRule ^([\w-]+)/?$ $1.html [L]

You can use this :

RewriteEngine on

RewriteCond %{REQUEST_FILENAME}.html -f
RewriteRule ^(.*?)/?$ /$1.html [L]