Lord knows
1. No one knows (except for God). Lord knows how long it will take for my application to be processed. A: "How do you think you did on your exam?" B: "Lord knows!"
2. It is absolutely true, certain, or definite that; it is obvious or clear that. Lord knows I need to do well on this test or I can kiss my scholarship goodbye!
Lord knows (that) I've tried
It is the absolute truth that I have tried my best (to do something). A: "You and Jeff just need to spend some time together and bond." B: "Lord knows I've tried. That boy just isn't interested in doing anything other than playing those damned video games." A: "You need to convince her to accept the deal." B: "Lord knows that I've tried, but she is adamantly refusing." A: "Then try harder!"