Outside is ever changing and tying your happiness to it means you are at the mercy of things not under your control. Find the buried flow of joy inside, that is constant and independent of all of life's up and downs.
When you stop chasing happiness by buying more, consuming more, possessing more and doing more, you can use your energy to actually create it.
Are your thoughts filled with gratitude, abundance and joy or are they about lack, fear and greed? The first will fill your life with happiness the latter with insecurity and dissatisfaction.
As Steve Jobs said "You can't connect the dots looking forward, you can only connect them looking backwards. So you have to trust that the dots will somehow connect in the future". Surrender to the journey.
Something I learned just last night, was that research has shown, if you engage in a behavior and then feel shame you are more likely to repeat that behavior. So if you break your food plan by eating that donut, or you have that drink you swore you wouldn't have, try to understand the behavior rather than beating yourself up - this is the way to success.
(I heard this in an interview with Kelly McGonigal author of The Science of Willpower).