long-cherished dream
A dream or ambition held for many years. Often used when that dream is finally realized or spoken of with deep feeling.
He became a novelist, which had been his long-cherished dream.
My long-cherished dream came true and I was able to open my own shop.
Compound of 長年 (many years) + の (possessive particle) + 夢 (dream). The phrase is transparent and follows standard Japanese grammar.