Confused. Someone cursed you or you wish to curse someone else? This course is eternal?
Here's the thing with eternal [generational] curses, I don't think they're a thing. I mean, maybe you could curse someone and it last a long time, but the energy will run its course if you stop feeding it, so either you or the person need to continually focus negative energy into the situation. With generational curses, it becomes a legend told in the family, and when bad things happen you're told 'see? Told you the families cursed' which results in negative energy being fed into something that may not be there or perhaps would of dies off of people stopped feeding it.
Personally, while I don't grown on curses and feel they have their use, you should think before you act. The curse can be cleansing for some [cursing their attacker for example] but most curse because someone stole their boyfriend or something that hurts right now, but in 5 years won't matter. [of course, you do you] so if you choose to curse some ex friend who wronged you, why would you curse their whole family? Seems very bitter and will cause you a lot of harm in the process [not saying karma, but you will be walking around thinking about how they hurt you a lot longer than you would if you tried letting go of the pain]