The hardest part of yoga, for me, is not "standing forehead to knee" (though let's be honest, I'm years away from that pose). The hardest part of yoga is the mental challenge, to keep my thoughts from running away and staying in the moment.
Here's an example: The instructor says "you want to get your forehead to touch your knee" and I think... forehead to knee, forehead to knee, almost there, this would be much easier for my friend C- because she has such a long, elegant nose, she wouldn't have to bend as far, they sure are a nice family, they're going to miss their two oldest kids when they go on that trip this summer, I hope their plane doesn't go down, losing a child would be unbearable, I can't imagine how they'd recover if that happened, the grief would be crushing.
And just like that I'm not in the moment and depressed about things that have not happened and on the brink of tears.
Here's another: This class is going great, I'm in the moment, I'm happy, yay for me, life is beautiful, life is beautiful... Life Is Beautiful was a really good movie but man it broke my heart, what the father did for the son and I don't think I'll ever get the image of the smile on his face as he marches to his death out of my head.
And there I am on the mat getting choked up about the Holocaust.
So that's my big challenge. Staying in the moment, staying positive, and keeping thoughts of suffering and death out of my practice. By comparison, "standing forehead to knee" doesn't sound so hard, after all.