"Don't stress out" can be followed by many different types of prepositional phrase. You can give a specific cause of stress ("over the exam", "on account of the interview"), but you can also refer more generally to a stressful situation ("in college"), give background causes of the stress ("from lack of sleep"), provide another characterisation ("without a good reason"), or give a duration, time phrase, etc ("for long", "when you're young"). The preposition will depend on the meaning, and this question is too vague to give a proper answer with every possible preposition. --Stuart F.
I think it's a shame when a question remains unanswered for more than a week because someone answered it with a comment rather than an answer. So I cut and pasted Stuart F.'s comment here as what seems to me to be a great answer. I take no credit for the answer itself.