A low-level thief goes back in time to alter his family’s life for the better, but discovers his changes have created a new timeline where his wife is dead. As he attempts to reset the timeline, he’ll find himself hunted by his powerful alternate self who’s determined to hold onto this new future.

The math on this is way too complicated and at the moment, I don't know how to simplify it.

I suggest that the reason he goes back in time should not be for himself but for the love of his life; he wants to become a winner who can offer her the life she deserves. Only to discover his new life as a winner entails losing the one thing he loved in his old life. Be careful what you wish for.

The math on this is way too complicated and at the moment, I don't know how to simplify it.

I suggest that the reason he goes back in time should not be for himself but for the love of his life; he wants to become a winner who can offer her the life she deserves. Only to discover his new life as a winner entails losing the one thing he loved in his old life. Be careful what you wish for.

The plot is unclear as a result of the structure in the logline and cluttering details.
"A "Never Try, Never Fail" type?" - is a strange description for the MC can you use a shorter and more specific description? Perhaps describe his job or position in society as well.

"... is given the chance to alter his mediocre life when he is visited by his future self,?" - If the inciting incident is his future self visiting him, there is no need for the first part of this sentence. You can start the logline with: After being visited by his future self? then give a short character description and get into the action.

"?they'll only have 48hours to reset the timeline before Earth's fate is sealed." - there is no direct cause and effect relationship between this goal and the inciting incident. When I read "...is visited by his future self?" I strait away thought, as previously mentioned, Bruce Willis from "Looper" or Biff from Back to The Future. But reading about sealing Earths fate was entirely out of the blue.

Lastly the stakes are not clear, what does "...before Earth's fate is sealed." mean? Will the Earth blow up? Will the people of Earth be taken over be war lords? What is the big bad thing the MC will fight to stop?

Hope this helps.