Which are the most difficult songs to sing?

I'm With You (the bridge and final choruses)
Anything But Ordinary (the chorus)

My Happy Ending (the bridge)

Runaway
When You're Gone (bridge)
Keep Holding On (chorus)

What the Hell (chorus)
Smile (chorus)
Alice (chorus)

Rock N Roll (chorus)
Here's To Never Growing Up (not a lot of room for breathing on the chorus)
17 (chorus)
You Ain't Seen Nothin' Yet (the yeah yeah yeah part)

But most of these are difficult for me to sing because Avril has a higher voice than I do. I'm an alto, which is the lowest female voice type, and Avril is at least a mezzo or possibly soprano, which are higher. If I sing most of her songs in their original keys I have to strain to reach the higher notes, even on a song as simple as Complicated. So for a lot of her songs I take it down a key or 2 and that helps a lot. If I had to sing a lot of her songs in their original keys I'd say all of the above are difficult but if I can sing them lower the only ones I find really difficult are songs like RNR and HTNGU that don't have a lot of room to breathe on the chorus.

Аlso

Hot (after the bridge)
Fall to Pieces (the high notes at the bridge)
I Can Do Better (the chorus, especially the last one)
Innocence (the high notes during the bridge)
and maybe Push (the last high note in the bridge is kind of hard)

I also used to not be able to sing the chorus to Together but now I can do it without much effort. The more I practice singing something that I find hard the easier it gets. I also find it very difficult to sing any song where Avril uses her beautiful head voice. You know, that yodelly thing she does, like in the chorus of Give You What You Like, Things I'll Never Say's bridge, at the end of My Happy Ending, etc. But that's just a personal weakness.

Goodbye and Falling Fast are too high for my male vioce

For me at least, I'm With You remains territory that I can't step into, and to me it just sounds wrong when I change the key.