It can mean you lost an item, e.g. "I lost my bag in the park", or "it" can be your emotional self-control. So "I lost it in the park" could mean, depending on context, "I started laughing uncontrollably in the park", "I started sobbing in the park" or "I got really angry and started yelling in the park".
I thought of this too. I just wish I knew how to say it in Esperanto. My dictionary does not have an English translation for "to lose one's marbles." :)
Gxin estas tre malgxoja, mi amas mian posxtelefonon kaj me ne povus vivi sen gxin :D
to find and to lose are not real opposites although they are often connected. The opposite of finding is not to find, in a certain sense to miss.