▲one day 的意思大家比较熟悉，意思是“有一天”(可指过去或将来)。如：

One day the temperature was 30℃. 有一天温度达到30摄氏度。

You will have to answer for your crimes one day. 你总有一天要因为你的罪行而得到报应的。

另外，要注意 one day 与 another, the next 搭配使用的情形。如：

His views lack consistency：one day he’s a conservative, the next he’s a liberal. 他的观点缺乏一贯性：时而保守，时而开明。

One day he wanted his lunch early, another day he wanted it late. 他一天要早点吃午饭，另一天又要晚点吃午饭。

另外，还要注意，在特定语境中，one day 并不是习语，而是自由搭配，意思是“一天的时间”，与 two days, three days 相对应。如：

You can meet several such people in one day. 一天之中你可碰到好几个这样的人。

The holidays are practically over; there is only one day left. 假期差不多完了。只剩下1天了。

He says he has written the report in one day, but I can’t believe it. 他说他在一天之内写完的这篇报告，但我不相信。

▲day one 的意思是“第一天”“开始”，与the beginning 同义，通常搭配的介词有 from, since, on 等。如：

This arrangement has never worked from day one. 这个安排从一开始就行不通。

The police knew from day one that I was innocent. 警方一开始就知道我是无辜的。

On day one of the election campaign the National and Liberal Party leaders were neck and neck. 从竞选活动的第一天起，国家党和自由党的领导人就难分高下。

▲你的句子意思：

I have been wanting to write to you since day one. It hurts that you cannot see this. 我从一开始就一直想给你写信，你不明白这一点真是让人伤心。