Idiom Request – Informal Version of ‘Now If You’ll Excuse Me’

You lose your place as in

I lose my place rather easily when reading, i'll end up missing some lines which have important info in them.

You can lose your place two different ways: you can lose track of what word you are on at any point, or you can lose track of what line you are on when moving from one line to the next.

About forgetting what you've just read because you lose focus or something cut you away from the story I think you can say lose track of the story like the line as you mentioned in your note. When you lose track of a story, you forget it. As in,

You’re reading an article online. After a few seconds, you can’t remember what you’re reading. You’ve lost track.

You might be looking for perfectionist, overachiever, or hard-liner for a noun.

Uncompromising would work as an adjective.

As for idioms, this could work:

He doesn't know how to lose. (Implies that he literally hasn't experienced loss, he doesn't know how to act appropriately when he does lose, or that he is incapable of not giving everything to win)

Different words would be used in different contexts; for example, a "hard-liner" is usually used in reference to politics or other ideological stances. The point of view is also very important, as "hard-liner" is typically used to describe someone you don't agree with and/or criticize rather than one of your allies or friends.