Am I stupid for almost losing my iPhone?

No you're not stupid. You left your phone somewhere one time. That's all. Want to stop this happening? Stop with the negative self talk. By which I mean the "I'm so stupid" "this always happens". If you go round telling yourself that, of course you'll end up doing something dumb. Any time you start thinking that way, pause a minute and think that situation over. You didn't do that because you're stupid, it happened because you were in a rush, or preoccupied, or something. And we all do things like that now and then, so don't beat yourself up about it.