"The thing is that I have tried so much to find a good balance, body posture and breathing that I am really confused at the moment and all the flute teachers tell me about it. It is about time for things to change! Could you please suggest to me a method and/or a place to begin?"
My response: Everyone has an opinion about posture! I'm sure you hear many confusing and contradictory things. What I like about the Feldenkrais Method is that there are no rules about posture, and there is no one good position. Feldenkrais sees posture as a point of equilibrium and uses going in and out of balance to find the place of balance, which can be different every day. My favorite flute teacher, Liisa Ruoho at the Sibelius Academy in Finland, says "You have to lose your balance in order to find it."
It's important to find your own balance and comfort from the inside, exploring different movements until you find what suits you. Everyone has a different shape and a different history and different habits, so one teacher's solution may not work for you. Also teachers tend to simplify and turn advice into rules. For me following rules takes me out of myself and causes more tension.
I recently taught the Feldenkrais lesson called "What Is Good Posture?" and recorded it. I'll post it now on my blog so you can do it if you'd like. It's also published in a book by Feldenkrais called Awareness through Movement. This book has 12 sample lessons which are a good introduction to Feldenkrais. There's another good one concerning breathing, which I have posted here.
Disclaimer: this is a recording of a live lesson, unscripted, and responding to the people who attended the class. It is not a substitute for a live class, where the teacher can observe your movements and respond to your pacing. But it's a good introduction to the method.