My observation is that the idea of happiness as an object that can be created and or found is the cause of a great deal of unhappiness.  Happiness is inner realization that may or may not be related to anything particularly happening in the moment. Thus, every moment has within it the possibility for happiness, even the crap ones.

That said we work for that which no work is required.

Meaning most of us need to learn how to open ourselves to the experience of happiness and flow with life vice resisting it. Ironically though we might desire to be happy for reasons we may not even be aware of we tend to resist it.

A first step may be getting a handle on how we judge, label and measure our expectations on how ‘life should be’. Are the Should’s the ego’s attempt to control life? Are the ways we imagine and measure happiness reliable and valid? Have we set ourselves up for failure or success?

For example, in your title ‘how to be happy alone’ the assumption could be that being in relationship made being happy easier. Did it make it easier? Why? How? In relationship did you place the responsibility of your happiness outside yourself? Perhaps you put the power of happiness into the hands of your partner? Was that fair?

Is it possible that the way you framed the question is unintentionally creating resistance to that which you seek? Is it possible that the ability to experience happiness has nothing to do with being married or single? Yes… Now you can start

Lots of helpful advice on the site about how to change ones thinking about happiness

The Art of Being Happily Single