I have passed this mantra along to my son.
My mantra is “What the fuck?!?”
I didn’t make some conscious decision to make it my mantra, though. I just say it all the time.
My sig line. ( Every time someone ask me how I’m doing, I give them this line. It brightens many a day.)
“It is what it is,” which I picked up from my boss, oddly enough. Kind of implies that your own actions and reactions are the only things definitely under your control.
Oon yellimon. Oon yellimon. Oon yellimon. Oon yellimon. (repeat)
Why not? and Tomorrow, tomorrow it will be better. (not to be confused with tomorrow, tomorrow, I love ya, tomorrow, you’re only…)
Nam myoho renge kyo.
or
That which does not kill us. . .
or various song lyrics from years past.
Nerves like nylon. . . nerves like steel (repeat ala Bauhaus)
Or, I am , I am. I am Superman. And I can do anything.
Or words to King of Birds. REM dork.
Meanwhile, my working motto, to get my shit in gear, is from Mencken.
“Every normal man must be tempted, at times, to spit on his hands, hoist the black flag, and begin slitting throats.”
Paraphrasing a Who song:
This is no crisis, this is you having fun.
This is no crisis, just another tricky day for you.
I have one at work: Me pagan por la hora.. . Spanish for “They pay me by the hour.”
Personal issues? “This too shall pass.”
I’ve been saying ‘mens sane en corporum sanum’ a lot lately, but it’s far from a mantra. I guess I dont have one.
“No worries.”
Makes every situation better, just keep repeating it. 
I have a genuine early 70s Maharishi Mahesh Yogi mantra, “personally chosen” for me at one of his TM seminars. They are meant to be meaningless and just a thought/sound to deflect intrusive thought. Mine is “ima” like “i’m uh”.
om mani padme hom (Jewel in the lotus) is the basic buddhist mantra. All over Tibet. There are some other tibetan mantra’s I’ve learned but the mani one is most popular.
I’m considering adopting “Ninja will overcome.” Exclamation point as necessary.
“Progress in life.”
For me it’s a good one when things look bad. Looking back there has always been steady measurable progress in my life. My thought is that as long as there continues to be progress, little bad things in the moment can’t affect me too harshly.
“I hate myself”
Probably not healthy, but it’s increasingly apt.
“Don’t be stupid. Don’t be stupid. Don’t be stupid. Don’t be stupid.”
I don’t know if it’s a mantra, but I say it to myself all the time.
“Whatever works.” That pretty much sums up my life.
“The secret to life is enjoying the passage of time.
There ain’t nothing to it, any fool can do it.” —James Taylor
That has been mine for a very, very long time.
Lately I’ve been trying out, “Hahm Sah”, sanskrit for “I am that.” To be employed whenever you encounter the power and beauty of the universe. Remind yourself that you too, are that.
Last year I came home and found a cicada emerging from its shell. I had never seen this process before, just the cast-off shells. I took a picture and went inside for a while. An hour or so later I came back outside and saw the cicada just before it took off. It was emerald green and had the most exquisite wings I had ever seen. I took a picture of that and sent it to a friend who was going through a divorce with the message “find your wings.”
Flash forward a year later. I had gone through my own rough patch and adopted “find your wings” as my own mantra. It really means “be the cicada” though. As I said to some of my friends later, I’d rather be a cicada than a butterfly. Cicadas are more assertive.
I didn’t save the pictures, though. I should really email my friend and see if she has them still.