Learn two words: regret, remorse

regret, verb, /rɪ'ɡrɛt/

1 

to feel sorry about something you have done and wish you had not done it.

Don’t do anything you might regret.
I regret leaving school so young.
He was beginning to regret that he’d come along.

2 

[not in progressive] used in official letters or statements when saying that you are sorry or sad about something.

We regret any inconvenience caused to our customers.
I regret that I will be unable to attend.
I regret to inform you that your contract will not be renewed.

regret, noun, /rɪ'ɡrɛt/

1 

sadness that you feel about something, especially because you wish it had not happened.

I have no regrets about leaving.
She has already expressed deep regret for what happened.
I decided with some regret that it was time to move on.
It is with great regret that I must decline your offer.
I lost touch with her, much to my regret.

2 give/send your regrets   

to say that you are unable to go to a meeting, accept an invitation etc.

My father was ill and had to send his regrets.

remorse, noun, /rɪ'mɔrs/

a strong feeling of being sorry that you have done something very bad.

Throughout the trial, he had shown no remorse.
She felt a pang of remorse for what she had done.
Filled with remorse, Dillon decided to resign.

remorseful, adjective, /rɪ'mɔrsfəl/

remorsefully, adverb, /ri'mɔ:sfuli/