For me, happiness runs hand in hand with contentment. It's when I still look forward to my hubby of 20 years coming in the door each night. It's sharing laughter at stupid stuff and it's about understanding and accepting each other's needs. It's about never having to fight to be understood. It's when we paint, he enjoys the rolling and I enjoy cutting in edges....we compliment each other.
Happiness also comes from inside. I have to be content and happy with myself to be happy in my relationship.
Finding happiness in love is about finding the right partner. Finding someone who isn't a challenge to accept and who suits all our needs. And who's needs don't run in opposition to yours.
Making happiness last is about each person being considerate of the other's needs.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.