C.P.E Bach Solfeggietto

This piece of music, is perhaps one of the most challeging things Ive played so far in my years of band because of certain notes, that are still out of my register, and I cant quite master. This notes are on the fourth phrase of the song, which are an E, G, F, of the highest register, that is listed in my book. I think that this notes are hard because of the position of the fingers, which is still weird for me, because I can move them fast enough to reach to the note on time, so there is a lot of squeaks when I play this notes. I still cant get my posture right, so that also influences the fact that I cant play the notes. I have noticed that if I sit a little bit forward in the chair and my back is in a good position the note starts to sound a little bit better, but I still have to work on those with patience, note by note, until the sound is fine, and not squeaking. Another thing that is challenging in this song, is the fact that everything is slured, this makes playing harder because even though you can just play the notes in a continous manner, the sound has to be right, and moving the fingers is "harder" because you dont have like a steady harsh beat that just makes you move the fingers with. The problem with the slurs, is that the notes have to be together, but have to sound even, like you just cant play less of a note because it is slured or if it isnt, they have to be even. So far I have been able to work out the first 3 phrases, and then skip the fourth, and move on to the ninth phrase, they are worked out note by note, I still cant played them in a slur because Im still not used to playing them fast or together. I will probably be able to work them out, if I sit with patience and do them over and over again, so that they get their proper sound and tone quality, for this I still have to work on my posture. I am setting an objective that by teh end of the third quarter I am able to play it completely, so for this I have to practice almost every day, sit down and get the upper register notes perfectly.