The last game I bought, took about 2 days to download. Thus my empathy for your situation is rather limited. That was on Steam though. If you have problems with mega, you should complain to the developer or look if a smaller version is hosted on itch. Itch did have it's own download problems recently, and there is no indication in your title or op that this was about external download link. If the game is hosted and purchased on itch, there will be an alternative download link, in case the regular one does not work. But the external links are controled by the developers. Some even use google drive, and iirc that can shut out all downloaders after a certain quota per month.