I shouldn't have done it.
I regret doing that.
I regret what I said.
I should have known better.
I feel bad about it.
I shouldn't drink and drive.
I wish I had never met you.
A: Hi, Tony. You look unhappy. What's wrong?
B: I made a big mistake.
A: What happened?
B: I really wish I hadn't done it.
A: What on earth are you talking about?
B: I got caught cheating. I feel so ashamed. The teacher saw me and told me I failed.
A: What were you thinking?
B: You know my father. If I fail, he'll kill me. I have to do well.
A: But what you should do is study hard.
B: I know, I know. It's all my fault. I feel awful that I didn't study and I cheated and I got caught.
A: So long as you learn from your mistakes.