When you work fixed price, you need to have a fixed scope.
It is completely normal to have discussions with the customer on the scope of the project. the key is to see this negotiation as a way to ask more money from your customer.
I have the impression from your description that at the start of the project you did not define the scope in detail and that now you battle with the interpretation your customer makes from your scope description and your interpretation of that same description. Every time you hear your customer saying something you did not expect, you need to formally put on paper that that was not your interpretation and that your customers interpretation will be seen as a (scope) change request and will result in a new offer (= added cash)
so you should stop coding and start better defining what the scope is of the work that needs to be done.
(and always make a contract with your customer that states he can not ask more damages than the contract value)