If it's hard work - then you married the wrong person (unless, of course, they are unwell).
I found my first marriage exhausting, as I was always making a great effort to be the person he loved - and that just wasn't the real me at all!
When I left, the clouds lifted, the weight fell from my shoulders and I was truly happy alone.
I met somebody who seemed totally unsuitable (we had absolutely nothing in common) and we just clicked, as two independent, self contained, complete individuals.
We got along like a house on fire as we enjoyed being together, yet didn't 'need' each other, didn't rely on the other - and weren't even looking for a partner.
So, second marriage, 21 years, 4 children, endless carefree happy days (except the last few years of illness, just the price you pay).