Is taking a ssri safe for 20 or thirty years? does it cause memory loss?

Typically, yes: If you continue to benefit from the SSRI and are not having side effects, I would consider it safe to continue, even for 20 or 30 years. If you are starting to wonder whether it is still helping, however, I would encourage you to discuss that with your personal physician. In my experience, there is nothing to indicate that SSRI's play a role in memory loss.