Stuck on overtones?

Hi guys,

I am currently a senior in high school and have been playing sax since 6th grade. I just recently tried to start working on my altissimo range. I have Rascher's Top Tones for the Saxophone and have been trying to work on the overtones for about a month now. I can get the Bb, the Bb an octave above that and the F, but not anything higher. Same goes for B, C, C#, and for D I can only play the D an octave up. I have been trying to get notes higher, but it never works and it is getting very frustrating. Any advice on how to perfect my overtones? Also, how long did it take you guys to develop your altissimo range? I want to play the Creston Sonata for Solo & Ensemble, which is in two months, but I heard there's a high G in it and I don't know if I will have learned it in time...