Try using this shortcut:
CTRL-^
Vim documentation :help CTRL-^:
CTRL-^ Edit the alternate file. Mostly the alternate file is the previously edited file. This is a quick way to toggle between two files. It is equivalent to ":e #", except that it also works when there is no file name.
And :help alternate-file
If there already was a current file name, then that one becomes the alternate
file name. It can be used with “#” on the command line |:_#| and you can use
the |CTRL-^| command to toggle between the current and the alternate file.
However, the alternate file name is not changed when |:keepalt| is used.
An alternate file name is remembered for each window.