Mahsa's answer is good, but I'd like to point out one thing.
평생 doesn't mean forever but "for life", "throughout one's life", etc.
"Forever" is most commonly 영원히 or 영원토록. There's also 영구히 and 영영.
So 평생 친구 means "lifelong friend".
평생 교육 is "lifelong education", 평생의 일 "lifework", and 평생의 후회 "lifelong regret".
그녀는 평생(을) 독신으로 살았다 means "She lived her whole life unmarried", for example.