Thank you very much for all your answers
Ben, I am going to try to focus only on some concrete aspect of breating, as you say, not on the breathing as a whole (maybe that's why I currently find it hard to seaprate it from heartbeat) Only on some of the many components of the breath. When anapanasati sutta says "body breath" I should have understood that body doesn't refer as a single entity but as a compound made of many parts.
Phil, thank you for your advice. It makes sense to use the heartbeat as far as it makes me stay "tuned" with what I'm doing, and not to worry so much.
About my practice, I've said that I can be aware of my breathing for hours, but not actually doing formal meditation. I am not able to sit down for more than 20 munutes because I can not bear with the pain on my legs, but I can do many things during the day while being aware of my breathing (reading, working, talking, walking, even listening music...). Maybe this is part of the problem: I am used to do many kind of different things and follow the breathing at the same time. Perhaps this attitude keeps me very far from achieving a Nimita. It makes my mind to be continually dispersed, not focusing on a single point.
About the technique, I have read many books with different systems, but for the last year I've been trying to follow the steps of the anapanasati as described by Buddhadasa Bhikkhu at the book "Mindfulness with Breathing", mixed with the advices of Ajahn Brahm's "Mindfulness Bliss & beyond". Maybe it's too much for me, I haven't gone further than the first tetrad.
About finding a Teacher, I'm currently living very close to Wat Wiwek Asom in Chonburi, but nobody can speak English. I think that the closest Wat with western bhikkus is Wat Boonyawad. I hope that one of this days I can get a car to make a visit there.
Thank you very much