I believe its all about being comfortable in one's own skin. To live with your own thoughts, to reflect without being distracted. To look deep, within, and fully "see" yourself.
I think, to be content "alone" is one of the hardest things to do, but also the simplest and most rewarding.
First, in our thoughts we must accept both our accomplishments and failures. Its like a "Come to Jesus " moment with one's self. We accept we are not perfect. Accept we have had failures. Accept we cannot change things in the past. Bottom line we forgive our shortcoming. And (this is the biggie) then we learn to love, with understanding, and compassion our own selves.
Then start doing those things that you have put off because of other obligations. I read, write, research, sing, dance........and its just me. I do whatever I want, No time frames.
I now found my life is rich. I would never replace the time I spent with my hubby or raising my kids, or on my career. My goals have just shifted, taken another path. I am content to be in my own company as well as with family and friends. Each one of us must finds the formula that works. But the key to that formula is love and acceptance of who and what we are.