Krithika,
Guess we haven't got our point across to you.
All your posts are stored in the site. They are
not removed from the site.
Only when you try finding them through the find posts / threads links in your profile page, the system does an automatic query to bring only the latest 500 posts and not the older ones. To keep the search query return result faster than taking all the time and resources making the site slower.
If you want to look for your older posts, you need to search by keywords that you remember to have used in the posts based on the topic.
Other ways to keep track of your posts and the replies received for them are:
1. Subscribe to your threads for email notifications - instant or daily or weekly notifications (Refer this
FAQ for subscribing to threads)
2. Having received recent replies, the posts will automatically show up in Today's posts or New posts when you browse through those menus everytime you login.
Hope this clarifies.