Are you familiar with those battles in which you play just for the "play". In those battles the loss never hurts because in the first place you didnt really care...
On the other hand, when you plan each battle as if your life depends on its outcome ... that's a different story...
My most sting loss happened long time ago... I think it was the second round of battles in a tournament. I did pretty good really maybe not the first place but i think i was top 5.
I got high hopes. And i did give my soul for each battle. Especially for a very specific battle:
It was among the last battles. My opponent did have strong cards but I noticed that he tended to repeat the same strategy over and over again. With my great genius I built the perfect team to defeat the opponent ... I was ready for victory.
Indeed I would probably have won if I had only remembered to consider the rules of battle which included poison. I will spare you the details just saying that I discovered the existence of the poison only when we were already in battle ... it was a humiliating loss - most of the time I know how to lose with dignity ... but in this battle I just felt like a complete noob...
Lesson - always double check the rule set of the battle.