Thanks a lot for your advice on how to solve this problem. Of course, none of your suggestions (yawning, U-blocking -which, incidentally, looks like contortionism) is working so far, but I think it's a matter of practice, so I hope I'll be able to play the hight notes before too long. A face-to-face teacher, as the Iceman suggests, would be a great solution, but I don't think there is a single harp teacher in my town.
If I may ask, Michelle (well, or any other person here who's also played a brass instrument), as a tuba player, have you found any correspondence between that instrument and harp regarding articulation? I mean, I've been playing French horn for nine years, and I was wondering, for example, whether the way in which I am used to play staccato might be detrimental to the way in which I must articulate notes in the harp (perhaps by stiffening my cheeks too much, or something like that).