Happiness is a choice
Thoughts produce feelings, for example, suppose you get information that your significant other is cheating on you - your feelings are quite turbulent with pain, anger, and betrayal - then a few hours later you find out that information was false and your significant other is completely faithful. Your feelings are jubilant, calm, happy, and relieved. It does not matter that you had different sets of information, or even whether any of the information was true, your thoughts got created from your interpretation of that information and then your feelings got created from those thoughts. That is a fact.
The important question is whether or not you can control your thoughts, and you can. The level of control over your thoughts is related to your awareness, and the practice of shifting your perspective and interpretation of the world around you. It is also possible to shift your thoughts by deciding to do so, by making a choice.
That last paragraph is packed full of stuff that is not easy to do, awareness, shifting perspectives, and letting go of judgments, concepts, and beliefs are very difficult and take practice and some help. Even then, we need constant reminders to practice what we have learned.
I struggle with happiness because I am hard-wired to worry (possibly through genetic predisposition, chemistry caused by my eating, exercising, sleeping habits, or my childhood development). Even if the hopeful outcome is likely and certain. This is a very bad habit and I have never been able to completely break it. But I have gotten better and better and one of the biggest tools that has helped me is repetition and diligence. The other is forgiveness.