"Christmas and whitened winter passed away, And over me the April sunshine came, Made very awful with black hail-clouds, yea
"And in the Summer I grew white with flame, And bowed my head down: Autumn, and the sick Sure knowledge things would never be the same,
"However often Spring might be most thick Of blossoms and buds, smote on me, and I grew Careless of most things, let the clock tick, tick,
"To my unhappy pulse, that beat right through My eager body; while I laughed out loud, And let my lips curl up at false or true,
"Seemed cold and shallow without any cloud.
Now the speaker describes the passage of the seasons: winter and spring and summer pass, and by autumn she knows that "things would never be the same."
She doesn't say how or why things wouldn't be the same, though. (There's a heck of a lot she doesn't tell us.)
She says that all the blossoms of spring "smote" on her – in other words, they made her feel bad instead of a happy and cheerful, like spring makes most people feel.
She became "careless" and depressed, and let time just kind of slip by.
She doesn't say that she was unhappy, but that her "pulse" was. Does she mean that she was sorry she had a pulse? That she was almost sorry to be alive? It's unclear.
She says that she "laughed" and smiled, but her heart wasn't in it – she allowed herself to smile whether it was "false or true."