hi

my main instrument is clarinet which i play in city orchestra, but from time to time they give my oboe to play. the first time i got it was like 3-4 years ago when i was just 12 and i remember that i got to play it ok in just about 3 months, then i had some problems with getting the new reeds so i got back to playing clarinet, and just a few days ago maestro asked me to play oboe with again, this time for permanent. ok, i know i`m boring with this story, but i just started to practise and i can`t go upper the G2 and i just can`t can`t indure more that 3-4 phrases in parts , my lips (mouth) just crack, and I can`t play, so if anyone can explain to me how to play upper tones (i mean by the TABS) i would be grateful and also if someone can explain to me in short the embouchure for oboe...and please give me support because i feel like a loser because i can`t play for more than 20 sec...

You need fast air not lots of air. Think that you are trying to blow out the candles on a birthday cake from ten paces away. Think of the speed of air needed to accomplish this task and try to duplicate it on the oboe.
Try to use an embouchure that is similar to whistling; chin flat and the corners of your lips in to form a very round O. Think of the same sensation while you play.
You will have trouble for the first while-but keep at it.