I can't do V ups?

I don't know what it is about them, I can't do them. I can do the 45 degree angle ones, where the legs aren't anywhere near almost vertical, but the hardcore ones where people pretty much fold themselves in half, I can't even do one of those. Any tips? I've seen them done many different ways and I'm sure theres no set correct way, but I'm mad at myself for not being able to do something so simple.