Restore posts, but only posts that exist in my database backup

You can import the old wp_posts table in your current database with different table name,

query and loop through it then update the current wp_posts post_content column from the old table where the ID matches.

Here’s an example SQL which you can directly do inside phpMyAdmin (this updates everything in single query)

UPDATE wp_posts SET post_content = (
    SELECT post_content 
    FROM temporary_old_wp_posts_backup_table_name
    WHERE `temporary_old_wp_posts_backup_table_name`.`ID` = `wp_posts`.`ID`
    LIMIT 1

/* just making sure  the ID exists on old post table ID*/
    SELECT 1 FROM temporary_old_wp_posts_backup_table_name
    WHERE `temporary_old_wp_posts_backup_table_name`.`ID` = `wp_posts`.`ID`

/* probably need a filter to only update post_type = post*/
AND  post_type="post"

Another options is to export the wp_posts from the back-up to a CSV file, loop and read each row and do the same as above (update the post_content of the current post table if ID matches with the id on your csv rows)

Remember to back-up your current database before playing with it. You can also export and import your current database in a new wp install which you can play before running it on live database.