How do I downgrade from Catalina to Mojave? When I get to the last step, my installer will not allow it.

I downloaded Catalina thinking that any upgrade would make my computer safer/better. Unfortunately after the three hour wait, I find that I am unable to use two applications which I depend on daily.

How can I go back to Mojave??? After I downloaded it, I am unable to install it.I get this message: This copy of the Install macOS application is too old to be opened on this version of macOS.

HELP!!!