I like living. I have sometimes been wildly, despairingly, acutely miserable, racked with sorrow; but through it all I still know quite certainly that just to be alive is a grand thing.

The quote is saying that while the speaker has experienced a lot of sadness and despair at times, they still know that there is something amazing about just being alive. Life can be a struggle, but there is still something amazing about having the chance to experience life. No matter how difficult things get, the speaker is still grateful for the ability to live.