At 15 i was told something about my life and thought about for 8 years now. so can stress be related to something you found out years ago. ?

Certainly: Emotional stress and related mental health issues can be caused by anything that bothers a person, regardless of how distant or recent that event. At its most extreme, consider ptsd, which is often triggered by events years or even decades in the past. If stressors are interfering with daily life, consider seeing your doctor for help.

Yes : It sounds like it was an important issue. You would probably benefit by talking it over with a psychiatrist or psychologist, since it seems like you're still suffering so many years later.

Slowly over time-yes: Things held close out of fear or shame, over time erode one's ability to contain them. Keeping secrets is always stressful but not always recognized until the stress boils over & has to "suddenly" be dealt with. Age 15 is vulnerable & acceptance so crucial that suppressing painful secrets is easier but destructive- even lethal. Uncovering the secret, like draining a lesion, is one step to healing.