I am also very much a beginner, playing since around September this year. I have been playing Native American pueblo style flutes for some time, which play somewhat similarly.
As to getting dizzy, I had the same issue, then I took a lesson from Scott August and he told me "Don't blow into the flute, breath through the flute!" Haven't gotten dizzy since.
As to playing flat, to support what others have said, the first time I played a shakuhachi in front of anyone, I was trying out one in front of Ronnie Seldin at the World Flute Society convention. He was very supportive, and one of this comments was that pretty much all beginners play meri (chin down, flat), and I should learn to play more kari (as others here have described). So rest assured that what you are experiencing is common.