It is very difficult to forget about old things, especially the past romances. Old flames hard die is said in the above situation. Actually the old flame refers to a person who you had a romantic relationship with in the past. old flame指的是你的旧情人。用火焰来表示爱人，这种比喻一定印象深刻吧。

Examples：

I bumped into my old flame in the shopping mall yesterday. 我昨天再商店里撞见了我以前的恋人。

Don’t talk about old flames before your girlfriend or boyfriend. 不要在你的男女朋友面前谈论过去的恋情。

You have to give her time. She doesn’t want to jump into a relationship quickly. You know old flames hard die.

你应该给她一点时间。她不想这么快地和前一段感情划清瓜葛。你知道回忆很难马上忘记。