How can I express how frustrated I am? Not about the game but about the customer.

He insisted to add one more stage an a bonus stage. That forced me to change some internals of the engine (my fault because I created a custom engine that isn't very flexible) so bugs arose everywhere. Now it doesn't work. Also people told me that Allegro 4 has issues with Windows 10 (some stuff don't work).

I think that I should rewrite everything creating a more flexible engine or find a yet created engine that I can use. Anyway I did a very bad work here.