We learn in parashath Bereishith that a man's wife is called his עזר כנגדו. It seems to me that Tehillim 121 is a perek that is uniquely suited to praying to HaShem to find one's soulmate.
In this manner the first two verses become: ...from where will my soulmate come? My soulmate comes from HaShem...
Just as HaShem created the heavens, where a couple is united in the root of their soul, He created the earth where a couple is initially divided, but will be reunited. This is hinted at in the last passuk, HaShem will guard your going and coming. Why is the going mentioned first? It refers to when the souls were divided and placed down here on earth. He guards not only the going, but also the coming together once more, the reunion of the divided soul down here on earth.