How to clear form cache automatically?
There are several method to clear the cache_form automatically.

Use optimizedb module 


  There is well-known issue with fast-growing table cache_form. This
  table stores Drupal form cache and it isn’t cleared when you flush all
  cache or run cron. Also you can struggle with problem of fragmentation
  with InnoDB tables on active delete/insert operations. This module
  provide functionality to solve this problem. Also its provide
  additional tools for maintenance database.
  
  Module OptimizeDB provide such features:

Ability to clean **cache_from** in administrative page or do it by cron.
Ability to optimize all database’s tables and display its sizes.
Configuration of notification about necessity to perform maintenance tasks.
Perform check and repair operation with tables.
Prevent crashing tables when perform all maintained actions.



If you have drush access  Safe cache_form Clear module


  Safely remove a limited number of items from the cache_form table.
  
  This module provides a drush command drush safe-cache-form-clear that
  can be run at a high frequency, keeping the table at a reasonable
  size. It requires that you use the database cache back-end for the
  cache_form table.
  
  Drupal's core cron can have problems clearing out the cache_form table
  when it grows extremely large. This module works around this by
  limiting the number of items expired from the cache in a single run,
  dealing with a manageable chunk each time.
  
  You can configure the number of items deleted in a single run by
  setting the $conf['safe_cache_form_clear_limit'] persistent variable.


for automatically clear the Drupal cache use  Cleaner module (it's not clear the cache_form table). 


  The Cleaner module allows the admin to set a schedule for clearing
  caches, watchdog, and old sessions.


and another solution is create a custom route in your module something that not predictable by others (some hash url), on call back function truncate 'cache_form' table (or equivalent Drupal ) then on your server or your Cpanel create  a crontab,schedule it to run the code periodically.