I should have...
B2Expressing a missed opportunity or a mistake.
I feel terrible about...
B2Expressing strong feelings of guilt or remorse.
I take full responsibility for...
B2Accepting blame for something.
Looking back, I realize...
B2Reflecting on past actions with regret.
It was a mistake to...
B2Acknowledging a wrong decision.
I'm truly sorry for...
B2Expressing genuine apology.
Please forgive me for...
B2Seeking forgiveness.
I have some regrets about...
B2Expressing mild regret.
With hindsight, I should have...
B2Expressing regret with the benefit of past experience.
I feel bad about...
B2Expressing feelings of guilt or remorse.
I'm ashamed of...
B2Expressing shame or embarrassment.
I could kick myself for...
B2Expressing strong self-criticism.
That was a poor decision on my part.
B2Acknowledging a bad decision.
In retrospect, I should have...
B2Similar to 'with hindsight', but slightly more formal.
I'm very sorry for...
B2Expressing a formal apology.
I failed to...
B2Expressing failure to do something.
I'm to blame for...
B2Accepting responsibility.
I messed up.
B2Informal way to express a mistake.
I'm at fault.
B2Accepting responsibility for a mistake.
I own up to my mistake.
B2Taking responsibility for one's mistake.
I regret to inform you that...
B2Formal way of delivering bad news with regret.
This is my fault.
B2Directly accepting blame.
I deeply regret my actions.
C1Expressing profound sorrow for one's behavior.
I have considerable remorse.
C1Expressing significant guilt and regret.