After many years of experience, I realized that losses are part of the proccess trading. Losses happen to the best traders. The best traders I know do not have a 100% win rate.
When I take a loss, I sit down in order to:
a) review the trade
b) learn from my loss
What do you do when you take a loss?
Well you are ultimately right, for as much as I know, losses have always and will always be part of trading, same way it is with gambling.
In any thing or activity where profiting totally depends on our ability to predict the future, or predict the outcome of something in the future, it is very normal that sometimes, or most times actually, we will predict wrongly, that is, our prediction won't be correct or accurate, which about a loss if it's trading we are into.
And also worth mentioning is that fact that it's not every loss that is a lesson to learn from, some or most loses(depending on your level as a trader) just happen naturally and there is nothing to learn from such, you just take it and move on to the next.. It's as simply as this.