Milton combines the ambition of youth and divine will through the wonderment of Time (notice that he capitalizes the word, too, which seems to personify it or to give it power).
The first line describes Time as a "thief of youth," which phrasing could represent the speaker's frustration or wonderment at how fast his 23rd year of life flew by. By the fourth line he is comparing his time of life with late spring which is a symbol of crossing a threshold from youth to manhood, but, of course, not middle age (autumn) or old age (winter). The speaker seems shocked that his age seems a lot older than he feels or looks in lines 5-6. The sense of confusion that the speaker has between what he sees and feels as compared to the time in his life continues in lines 7-8 when he discusses "inward ripeness" that doesn't seem to show on the outside to match the time of life.
At the beginning of the sestet (or the last 6 lines) there is a shift in attitude and voice. He seems to turn away from the clock and the mirror in his mind to realize that no matter what he tries to compare Time with (physical or emotional feelings) it doesn't matter because it's all out of his hands anyway. He realizes that he can't change or stop time, so he might as well use it well because Heaven's "will" isn't going to change for him. And the tone doesn't seem bitter, but accepting, of the fact that his "Taskmaster's eye" sees all and is in charge of Time.
In summary, Milton blends personal ambition with divine will as the speaker realizes his age and stage in life having come upon him more quickly than he had hoped, but recognizes his mortality with the fact that only Heaven is in charge of time.