Like Chasem said, Practice, practice, practice. Your lips aren't used to having to grip something, so the muscles in your embouchure are having to develope from scratch - think of it like working out, or running; you can only do so much at first, but with regular practice, you get better.

I play the concert flute. I'm getting back into it after a twenty year hiatus (I used to be very good), and I'm suffering from the same thing; after so long, my lips just lose it, and I can't focus the air for that upper range anymore.