You know those times when words that you don’t mean to say fly right out of your mouth before you can clamp it shut? You have an outburst at your spouse or children, and you wonder why it is that you’d never talk that way in front of a friend or sister-in-law, but find it so hard to self-regulate in the privacy of your home.
In searching through pages and pages of Jewish wisdom and guidance on this matter, I stumbled upon a piece of advice so piercingly short that at first, I brushed it off as being too simple.
In a private audience with the Rebbe, a woman described how difficult it was for her to overcome her moodiness, specifically her anger. The Rebbe followed up with a letter in which he wrote:
In answer to your question regarding anger, contemplate the verse from Psalms, ‘I have set the L‑rd before me at all times.’ These words are found also in the introduction to the Shulchan Aruch, which directs the behavior of a Jew in his day-to-day life. ... When one envisions that at every moment he is in the presence of G‑d, how is it even possible to be overtaken by moodiness and anger?
If I can control my impulses in front of another person, I most certainly can control them if I realize that I am constantly in the company of G‑d.
Thoughtstream: Today, I will be conscious of the fact that even when no one outside of my family is home, there is another Presence in the room.
(Adapted from Morah L’Dor Navuch, vol. 1, pg. 248.)

Thank you for this reminder! It seems that when we need help we something, we can find that the Rebbe wrote the answer somewhere. How wonderful for us to have the Rebbe's legacy.
I believe it is from psalm 16:8.
An instructor had told me the same thing when we were working on our Elul Avodah. But, why just Elul? As the Rebbe pointed out, we need to remember it at all times whenever we feel moody or angry, that we are always in Hashem's presence.