I'm really a romantic at heart.我骨子里很浪漫。

She's 81 years old, but she's still young at heart. [=she behaves and thinks like a much younger person]她81岁了，但骨子里还很年轻。

We have your best interests at heart. [=we want to do what is best for you]我们要做对你最有利的事。