This one has bugged me for a long time. Why would Hagar go back to a pressured existence especially when she was told that her son would become a wild man and a thief?
Although I am searching for the Midrash, I found a note years ago that the Midrash says Hashem was telling Hagar what would happen to Yishmael if she did not return to Sarah. Therefore, Hagar returned to her "pressured" life (i.e., profoundly meaningful, rather than misguided freedom to "do what I feel like") in order to give Yishmael the opportunity to grow in a loving, uplifting, and disciplined environment to curb his inclination from being a פֶּרֶא אָדָם to one who could (and would) eventually do teshuva at the end of his life.
The Ramban explains that Hagar would've never become free from her mistress.
RETURN TO YOUR MISTRESS, AND SUBMIT YOURSELF TO HER. [The angel] commanded her to return and accept the dominion of her mistress. A hint that [Hagar] would never become freed from her, because the children of Sarah would rule over her descendants forever.
Refer also to the Sforno, for a similair approach:
והתעני, the additional comment after telling Hagar to return to her mistress, was an allusion to the future; it would be Hagar's destiny, as well as that of her offspring to submit to the authority of the Jewish people.
See also the Tur in Tur HaAroch.
שובי אל גברתך והתענו תחת ידיה, “go back to your mistress and submit to her oppression” The angel hinted to her that she would not regain her status as a free woman, and that the descendants of Sarai would rule over her descendants into the distant future.
See also the Midrash of Philo where it goes a little bit deeper into the concept of obeying a mistress in this case:
Why did the angel say to her, "Return to thy mistress and be humbled beneath her hands?" (Genesis 16:8). As the letter is plain, we must rather investigate its inner meaning. The word of God corrects that soul which is able to be lured, and instructs it, and converts it, leading it to wisdom as its mistress, that it may not, through being abandoned by its mistress, rush at once into absurd folly. But it warns it, not only to return to virtue, but also to be humbled beneath its hands, that is to say, beneath its several excellencies. But there are two kinds of humiliation; one, in accordance with defect, which arises from spiritual infirmity, which it is easy to overcome, seize upon, and reprove. But there is another kind which the word of the Lord enjoins, proceeding from reverence and modesty; such as that humility which children exhibit to their fathers, pupils to their masters, and young men to the aged; since it is very advantageous to be obedient, and to be subject to those who are better than one's self; for he who has learnt to be under authority is in a moment imbued with a power which he alone may exercise; for, although any one were to be clothed with the authority of all the earth and sea, yet he would not be able to possess the royal supremacy of virtue, unless he had first been instructed and taught to obey.