I have a form with many fields. If I click the Discard button, the form get closed and all filled fields get removed but I don't want that. Suppose we have 100 fields in a form and refilling those is not easy.

but by introducing a new button how will you distinguish which one to keep and which one to reset ?
coming to your question, if you need a make a button clickable bypassing the required fields, there was some apps in the store and also using js it can be done similar to current discard button

about your question in the first line of your comment, we will have a function that calls when the button is clicked. In the body, we can clear mentioned fields.
But, the main problem is that when you push any Odoo button, before the actual method is executed the current record is saved (either 'created' or 'written'). It means that a check for 'required' and being filled is made before you fill the value in your method.