When people say "choose happiness" they're trying to make things simple. You don't actually get to wake up in the morning and have two buttons presented to you on a silver platter one reading 'misery' and the other reading 'happiness' and then frantically scan your brain as you try to figure out which emotion might be better for that day. Sure, you can wake up and tell yourself "today I choose happiness" but if things go wrong that day, then your perspective might change. Things happen, things go wrong, our expectations won't always be met, and life throws us more than a few curveballs. You can't just "choose happiness" and make all your problems disappear and have life go back to being peachy keen. (But if that button is ever invented, let me know so we can be the first ones to buy stock).
You can't choose happiness but you CAN choose to not compare yourself to other people. You can choose to stop looking at the Instagram explore page and letting edited pictures control your life. You can choose to pick up groceries and cook healthy meals for yourself. You can choose to go to the gym instead of going back to bed because you know you'll feel better after. You can choose to go to church on a Sunday and be uplifted and inspired by the message. You can choose to text your friends and family and just tell them that you love them. You can choose to not go through his twitter likes because, let's be honest, you know you'll never like what you see. You can choose to pamper yourself with an at home mani-pedi and a great playlist to soak in the bath too. You can choose to look at yourself in the mirror and instead of noticing flaws, point out everything God's blessed you with. You can choose to take a drive with no destination and a Starbucks in your hand. You can choose to not get frustrated when plans change and realize that the best things really do always happen unexpectedly.
The best things always happen when you're focused on your goals. The best things always happen when you discover something new, and look forward to the future, and buy yourself some ice cream every once in a while. The best things always happen when you're living YOUR life and not focusing on everyone else's, and when you finally say goodbye to things that are no longer a part of your plan. The best things are yet to come and you can choose to let them in. So yes, choose happiness, but choose it by doing what it is that makes you happy and running far far away from the things that don't.