Green Day - Whatsername Lyrics Meaning

I think it's about a guy ho let th girl he loved get away and he's nowwonderinghow she is now and what would have happened if he'd told her how much he loves her.

"Thought I ran into you down on the street
Then it turned out to only be a dream
I made a point to burn all of the photographs
She went away and then I took a different path
I remember the face but I can't recall the name
Now I wonder how whatsername has been"

He thinks he sees this girl that he loves on the street, but it's not really her. Then it reveals that they used to be friends or even going out, bu henever told her how he really felt about her and she left, so he's trying to forget about her. It's been so long that he can't remember her name, but the face has always been haunting him, and he's still thinking about her, woderng how he life is now.

"Seems that she disappeared without a trace
Did she ever marry old what's his face?"

I think maybe they were good friends,and though she was with someone else that he did't really know he still loved her. But she just left one day with What's His Face without saying a proper goodbye.

"Remember, whatever
It seems like forever ago
Remember, whatever
It seems like forever ago
The regrets are useless
In my mind
She's in my head
I must confess
The regrets are useless
In my mind
She's in my head
From so long ago "

This section is just bsically saying I remember her from long time ago, but regretting not telling her I love her is useless beause she's not here anymore, she's only in my head. And he's confessing that he still thinks about her lot.

"And in the darkest night
If my memory serves me right
I'll never turn back time
Forgetting you, but not the time.

When the pain of losing her is the worst, he tries to remember the good times they had. He realizes that he doesn't get another chance to tell her, so he's trying to forget her, without forgetting what might have been some of the best years of his life.