And when they recur, she is outside of girlhood.

One touch of the courage she shows in the last scene would have saved in the previous scene herself, her lover, and her brother.

The lie she lets her brother infer when she allows him to think that the lover she has confessed to is not the Earl, yet that she will marry the Earl, degrades her altogether and justly in her brother's eyes, and is so terribly out of tune with her character that I repeat I cannot understand how Browning could invent that situation.

It spoils the whole presentation of the girl.

It is not only out of her character, it is out of nature.
Indeed, in spite of the poetry, in spite of the pathetic beauty of the last scene, Mildred and Tresham are always over-heightened, over-strained beyond the concert-pitch of nature.