lifelong ：
adjective形容词
lasting or remaining in a particular state throughout a person's life
毕生的,终生的
the two men were to remain lifelong friends.
两个人会成为终生的好友.
always ：
adverb副词
at all times; on all occasions
始终；总是
the sun always rises in the east.
太阳总是从东方升起.
forever ：
adverb副词
(亦作 for ever)for all future time; for always
永远,永久
she would love him forever.
她会永远爱他.
三个词的词性和例句如上.
always和forever都是副词,要区别的话,只能看句子本身.
一般情况下,always多数用在句中,forever多数用在句末.