When the song said 'If you can't have the one you love, love the one you're with' well, they were wrong. That is not the key to happiness.
Loving yourself, and deciding when to let other people into your life is.
Once you're happy, you're happy. If you find someone that makes you even happier that is the bonus.
And the roughest times then, well, they don't feel like work. Just something that needs to be done, like laundry or lawn care. Gotta say the best times are out of the ball-park though.