Make the most of your regrets; never smother your sorrow, but tend and cherish it till it comes to have a separate and integral interest. To regret deeply is to live afresh.
Henry David Thoreau
This is truly beautiful.
It is so easy to say, "Oh, don't have any regrets," or even, "Don't regret your mistakes; you learned from them," or "...they made you who you are." etc. But we wouldn't be human without regret. Regret hurts, so we try to ease the pain by inventing platitudes, pretending not to regret anything or that the things we regret were really blessings in disguise.
It's a relief to feel allowed to grieve for a past you wish had been different.