Evermore means for ever — from now on, with no end. It is not the everyday word; people say forever in normal talk. You hear evermore in songs, poems, and older or churchy English: happy evermore, for evermore. Though actually, it is one word, and that matters, because ever more as two words is a different thing: ever more then means more and more, an amount that keeps growing. Evermore does not mean a larger amount; it means always, to the end. It feels formal and a little old-fashioned, which is why songwriters like it.