Hard Vowel

Hi, i was developed my mix voice almost 7 months, high notes with open vowel is nothing now. It's become very easy and effortless. But, i've got new problem is singing in my upper passagio with "O" (U sound) vowel. I'm practice singing for Indonesian song "Dewa 19 - Pupus" on the chorus is A4 note i can't open my throat to go into mix voice, i was always get swallowing sensation that forced my throat to closed. But, if i'm not using mix at all, i can still reach the notes clearly, eventough many or sometimes the notes become sounds flat and the volume louder.