Questions & Answers about Jag var mer stressad förut.
Both are past, but they focus on different things.
Jag var mer stressad förut.
Literally: I was more stressed before / earlier.
This is a simple past description of a state at an earlier time. It doesn’t say anything about a result now; it just contrasts past you with present you.Jag har varit mer stressad förut.
Literally: I have been more stressed before.
This focuses on your experience at unspecified times in the past (“at some point(s) before now”), and is closer to English “I’ve been more stressed before.”
In most everyday conversations about how you used to be compared to now, “Jag var mer stressad förut” is perfectly natural and probably the default.