Analysis of Somebody's Song

This is what I vow;
He shall have my heart to keep,
Sweetly will we stir and sleep,
    All the years, as now.
Swift the measured sands may run;
Love like this is never done;
He and I are welded one:
    This is what I vow.

This is what I pray:
Keep him by me tenderly;
Keep him sweet in pride of me,
    Ever and a day;
Keep me from the old distress;
Let me, for our happiness,
Be the one to love the less:
    This is what I pray.

This is what I know:
Lovers' oaths are thin as rain;
Love's a harbinger of pain-
    Would it were not so!
Ever is my heart a-thirst,
Ever is my love accurst;
He is neither last nor first:
    This is what I know.