"
I saw that day,
Lost my mind
Lord, I'm fine
Maybe in time
You'll want to be mine
The whole song is basically a tribute to the original Noodle because she was killed. Very sad, very tragic. I literally cried.
Anyway, " I saw that day, lost my mind" is what he is saying to describe the whole sad depressing day when they killed Noodle and that's what made him lose his mind and what caused his depression and sadness. (Mine too.)
He says "Lord i'm fine" to say that he is still hanging on, waiting for his happiness to come back and he still hasn't lost his faith in whoever his Lord is and he will definitely be around when and if it comes back.
When he says " Maybe in time, you'll want to be mine." he is saying that Hopefully in another life, whatever it is he really wanted will one day want him too.