To be honest 老实说

To be honest, I just don't want to go.

说心里话，我真不想去。

To be honest, I am disappointed at you.

说实话，我对你很失望。

To tell the truth 说实话

To tell the truth, I don't agree with you.

真格的，我不同意你的意见。

To be candid with you 坦白的说

To be candid with you, I think you're making a dreadful mistake.

跟你说实话，我觉得你在犯一个可怕的错误。