My personal thought, and it is just an idea of my own;

Happiness is a crazy target to aim for.

Happiness just like sadness, anger, pleasure, joy and anxiety are emotional responses and emotions rise and fall like the tide.

Myself, my aim is more for contentment.

When I am content I can have a great day and a truly happy moment, or an awful day and a bloody terrible moment, yet there's no great rollercoaster of emotion or expectation of any. I can accept each mood as it comes and passes because, over all, I am content day to day.

You've got the right idea with gratitude, most definitely. Generally I say thankyou every day I get to wake up and am grateful for another chance to live. There were times when that wasn't as certain as it is now.