Mhm...

I wonder what is the acceptable threshold for that, which once you cross you actually become the person with unacceptably low level of self confidence.

In animal world, losing a battle has long lasting detrimental effects on self confidence and accepting subordinate status is often the end result. It probably applies to humans as well in some measure.

That might mean there is either a threshold or milestone events which can turn the 'lose and you'll win' into 'lose and you'll more likely lose in the future'.

It might be that humans have many competitive domains, unlike animals, where each loser can start it again from the 0 and start rebuilding confidence.