[Aga-chan]

I'm not sure either, but I guess that "the beginning of everything" refers to the reality. For example: One lives in a (seemingly) happy relationship, so happy that it seems like a dream. Then one day the happiness and love fades away, and you become aware of the reality and what comes with it then (pain, despair).

To understand the lyrics it is important to know that it's based on kachoufuugetsu, which stands for the beauty of the nature in Japan. I think it portrays a perfect scenery. The word is written by the kanji for "flower", "bird", "wind" and "moon". You will find that quite often in Japanese art.
It was also used in the "part of Me" PV. With blossoms falling from the tree and I think there was a scene where birds flew away from a tree.

And as to the self-sacrifice: ayu used that thought in other songs, too. Like "Endless sorrow" or "BLUE BIRD". I think it just stands for protecting someone or something no matter what, which shows how important that person or thing most be for someone.

Those lyrics are really vague. Specially since I'm not sure whom she refers to in this song. I don't think I've been a help... Sorry!