I am supporting a RL friend through some shitty times - no real end in sight 
She said the exact same thing I recently saw posted by a battle-experienced, wise MNer 
:
"Keep putting one foot in front of the other, and keep breathing in and out."
Pace yourself.
Be kind to yourself and look after yourself.
This too shall pass I may have used that one rather a lot in my own head
Que sera, sera.
Let it go.
You cannot change somebody else's behaviour; you can only change your response to it.
I am sure there are many more.
They are all quite trite truisms, but not any less true for all that.
Please add your own - anything at all that kept you hanging on when times were tough and no light at the end of the tunnel was forthcoming.
And I do hope you did come out the other side and don't need your own mantras anymore.