I can confidently state that some of my happiest moments have been when I've been alone.  This is true for days where I've simply happened to keep my company, like the Sunday years ago when I had a Father Ted day.  Its also been true for times when I haven't been in a relationship and was able to live life how I pleased.

Being alone is great when there's nothing wrong.  If you're heart broken, or in financial difficulties, or ill in any way then its not good at all.  If you can remember a time, then, when you were alone and happy then its a reasonable assumption that your life was fairly trouble-free at that time.

I once broke up with a girl and was so relieved afterwards that my time alone felt like a holiday.  I spent a fair period of time enjoying the lack of her in my life.  Sometimes a person can alter things and it happens so slowly that you're not consciously aware of the change.  My life had become a warped version of itself and I didn't know how much it had warped until she was gone and it de-warped itself.

I can remember someone complaining that he had put on weight and he found it impossible to shift.  He was overweight for a long time until he, in an unrelated move, split up with his partner.  Within a few months the weight was gone and he'd not even tried to lose it.  Being in the relationship had changed his routines, exercise habits and diet so much that he had become overweight and unhappy.

Now, I'm not suggesting that your partner is to blame for any negative situations in your life.  Nor am I implying that your quality of life will improve if you end your relationship.  In fact quite the opposite might happen.  What I am sure of, though, is that sometimes its good to be alone... even for just a little while.