Not easy.. sometimes, depending on your outlook, do you consider it a mistake in the first place?
Easy: If to you, it was a mistake not getting your degree, go get it doing part-time etc.
Hard: If you think it was a mistake not having a good relationship with your parents, work on the relationship to improve it. Or a broken friendship which you later realised it's your fault, not your friend's.
First, a recognition of whether it's a mistake or not. Then, if yes, genuine sincerity to want to make amends.
Of course what's hard for some may be easy for others, and vice versa.
I've made and sadly am still making mistakes (that's life!)
Rectification doesn't mean you have to turn back the clock to undo, or doing something big to counter that mistake, sometimes it just being forgiven for the past mistake.
And to be forgiven is happiness rite?
Getting lor sor here.. people will think I'm really that old