How can I change my film from saying "Dog Eat Dog" (2018/I) to just "Dog Eat Dog" (2018)?

After searching for Dog Eat Dog
and choosing Exact Title Matches,
you get a list that shows the other titles of the same name in 2016
https://www.imdb.com/find?q=Dog%20Eat%20Dog&s=tt&exact=true

Please see https://help.imdb.com/article/contribution/titles/title-formatting/G56U5ERK7YY47CQB specifically:

If two films have the same title in the same year we add a "/" followed by roman numerals after the year as necessary. For example, Twilight (2008/I) and Twilight (2008/II).

This is all that has happened here.  The presence of the roman numeral has nothing to do with any issues with the ordering of your filmography (see later below). 

As bderoes helpfully notes above, see https://www.imdb.com/find?q=Dog%20Eat%20Dog&s=tt&exact=true where you can see there are two titles which share this exact name released in 2018:

Dog Eat Dog (2018) (I)

Dog Eat Dog (2018) (II)

so the roman numerals are essential per the rules quoted above. 

Back to the ordering of your filmography at https://www.imdb.com/name/nm9608894/ ... the reason Daddy Issues (2017) is listed above a 2018 title is because it has the production status of "(completed)" and lacks a release date.  You can fix this by adding details of where it was shown to the public in 2017 via the release dates section on https://www.imdb.com/title/tt7973930/releaseinfo -- adding the release date will flip the production status from completed to released. Titles in-production always take priority over titles which have not been released when ordering a filmography. 

Also on Dog Eat Dog, your release date on https://www.imdb.com/title/tt7969662/releaseinfo lacks details of how and where it was released ... we do not think you meant to imply your short had a nationwide release across all cities in the USA?