I am fortunate that my personality type is pretty-much the optimistic-happy-type, so it comes easier to me than some I suspect, but still it's not always easy even for me. So I guess the question I want to address here is - How to choose happiness even when it's not easy to?
1) Make time in your life to do things that make you happy.
I teach a lot on being selfless and giving yourself away, helping others, etc etc. But there is a balance to that! You can't give what you don't have, so it is vitally important to give to yourself!
2) If there is something in your life that is ripping happiness from you, change it.
Obviously there is a balance to this as well. I mean kids can rip your happiness from you at times, I know this cause I have 2 of them, ha ha! So I am not saying so post them on craigslist ... no no no! I am talking about those things that we sometimes feel obligated to but don't have to do. But since we are people pleasers we do them. I suggest you stop. :) If it's a person, then maybe there should be a change, or at least a heart to heart conversation.
3) Resolve your inner issues.
In my 24 years of ministry I have seen people's past issues cause many problems in their life. It keeps them from happiness, and it doesn't need to. Resolve the issues. And whatever you do learn to be a person who forgives and moves on! Don't allow junk from the past to dirty your future.
4) Find peace with God.
Of course I believe that true happiness is achieved through a healthy relationship with Jesus Christ. But let's just be honest here, I know, I am sure you do as well, Christians who are extremely unhappy! So I am not naive enough to think that just "being a Christian" makes you a happy joe lucky person! And when I hear preachers who try and sell that message I cringe, because it's just not so. It's just like the message of being a Christian will make you healthy and wealthy. Nutty! To be wealthy you must apply principles of wealth to your finances, same for health ... apply the principles. (I will stop ranting!)
I am aware that life isn't fair. Some people in this life are dealt poor hands and it sucks! And for them happiness can seem allusive, but it's worth the pursuit! Blessings to you!