Noticing Restlessness-&-Regret (uddhacca-kukkucca) arise can make it fade away:
Herein, Bhikkhus, when Restlessness-&-Regret is present in him, the bhikkhu notes & understands:
There is Restlessness-&-Regret in me, & when Restlessness-&-Regret is absent, he notes & understands:
No Restlessness-&-Regret is in me. He also understands how unarisen Restlessness-&-Regret arises.
He understands how to leave behind any arisen Restlessness-&-Regret, and he understands how left
Restlessness-&-Regret will not arise again in the future.                                                MN 10

What is the feeding cause that makes Restlessness-&-Regret arise?
There are unrest, unsettledness, nervous unease, agitation & anxiety, often giving irrational & unwise
attention to such states, this is the feeding cause of the arising of unarisen Restlessness-&-Regret, &
the feeding cause of worsening and exacerbation of Restlessness-&-Regret, that already has arisen.
SN 46:51

What is the starving cause that makes Restlessness-&-Regret cease?
There is the mental state of serene tranquillity, calm, quietude, rest, stillness, imperturbability, peace,
frequently giving rational & wise attention to this exquisite mental state, is the starving cause for the
non-arising of unarisen Restlessness-&-Regret, & the starving cause for the dampening and calming of
Restlessness-&-Regret, that has already appeared.                                                        SN 46:51

Some advantageous reflections to return to whenever Restlessness-&-Regret is provoked:
When the mind is restless, it is the proper time for cultivating the following factors of enlightenment:
tranquillity, Concentration and equanimity, because an agitated mind can easily be quietened by them.
                                                                                                                                               SN 46:53
Restlessness-&-Regret is like Slavery:
Just as when a man is a slave, not independent, but dependent on others, unable to go where he likes,
exactly & even so is restlessness since it forces one into unwanted activity & destroys any ease & calm.
Later he is set free from slavery, is now independent, no longer dependent, a freeman who can go
where he wants. And at that he rejoices, is glad at heart... Such is blissful freedom from restlessness.
DN 2

Deliberately Directing: Conscious & Clever Centre of Concentration:

 Herein, Ananda, a Bhikkhu attends to this Focus:
This is Real, this is Supreme, namely:
The Stilling of all mental Construction,
The Calming of all Restless Activity,
The Fading of all Concern & Anxiety,
The Cooling of all Temptation & Urge,
The Ending of all Longing & Craving,
The Exhaustion of all Fuel of Being,
Ceasing, Peace, Bliss, Freedom, Nibbana …
                                                                                    AN V 319
 

With all his attachments cut, with the mind's agitation quieted, calm and serene and happy is he, for he has attained peace of mind. Samyutta Nikāya I, 212	Calm is his thought, Calm is his speech, and Calm is his action, who, truly knowing, is wholly freed, perfectly tranquil and wise. Dhammapada 96
DOING GOOD = NO REGRET! Here and now the good-doer rejoices... Even so after passing away and re-emerging, the doer of good reaps only Joy & ease... So both here & there, the wise with merit well done, enjoys the purity of prior actions. Dhammapada 15	DOING BAD = MUCH REGRET! Here and now the bad-doer suffers... Even so after passing away and re-emerging, the doer of wrong reaps only pain and regret... So both here & there, the evil wrongdoing fool suffers the painful results of prior actions. Dhammapada 16

How to get to the opposite good mental state: Calm (Samatha) see here:
Forest Bliss: http://What-Buddha-Said.net/drops/Forest_Bliss.htm

See also:
http://What-Buddha-Said.net/drops/IV/Suitable_Substitution.htm