According to https://www.vocabulary.com/dictionary/bliss,
"Bliss is a state of complete happiness or joy. Marriage is often associated with this joyous feeling: people who are married and still in love are described as living in wedded bliss.
Another common association is heaven or paradise, as in eternal bliss."
I believe both bliss and happiness are highly related. Bliss may be the highest level of happiness.
Happiness is a feeling. It is a positive feeling. It is favorable and cheerful. A normal feature of a feeling is being temporary or temporariness. It does not last for a long time. In other words it is not permanent. When you get a salary increment as an employee you will get the feeling of happiness. After one or two hours, or one or two days that feeling of happiness gets finished. When you meet a loved one after spending some time away, you become happy. That happiness tends to diminish and then go from you within a short time.
Happiness is a sense of useful life. It is a realization of a purposeful life. If the meaning of happiness is a sense of useful life it is not a fleeting feeling. It is not a passing thing and that lasts for a very long time or forever (until the person lives).
According to Harris (2007), the word ‘happiness’ has two very different meanings: (1) a feeling: a sense of pleasure, gladness or gratification, and (2) a profound sense of a life well lived. According to Yacobi (2015), happiness is localized in time, as it is a fleeting state of mind, whereas the good and meaningful life is extended in time, since it also relates to the past and the future, and thus it is valued as a whole.
Bliss is eternal: happiness is momentarily. Bliss is the state of remaining present in the present and observing and feeling what is around without thoughts distracting you.