The Absolute Truth (sat) always exists and does not change whereas the unreal has no existence. You need to see the truth. If you don't see or realize the truths, then you should start with acceptance of truth with faith. Most important truths explained in Gita are:
- You (atma) are eternal, full of knowledge, full of happiness, complete, never changing and beyond inert matter and body. Your body dies but you never die.
- Atma is part of Paramatma (God). You are Paramatma's. Ultimately, you are one with Paramatma.
- Everything and everyone is ultimately Parmatma. Parmatma is in everyone and is everywhere. Everything ultimately belongs to Paramatma.
- As self (atma) is beyond matter, it is ultimately not the doer of actions. Actions are carried out by the nature (matter) in the nature (matter).
Right now, you may not realize above truths. You should accept these truth. Gradually, the realization will develop with the acceptance.
How to become happy and worry free? When you accept you as eternal and beyond body, then you don't give importance to negative events in your life. You accept things as they are. You let go things which make you sad and worried. You are satisfied and content and in harmony with your self, other people and environment.You will find things to be changed in the world to make it a better place. You act to help other but still, maintain your inner state of happiness. You are not worried or sad as self is beyond matter and is always happy. From the perspective of bhakti, you are part of Paramatma and He eternally loves you. Paramatma will always do good to you and will always maintain and protect you. You should be peaceful with this acceptance.
How do you act with above acceptance? When you accept that Paramatma is in everyone, then the tendencies of envy, rivaly, discrimination and abusiveness will go away and you will see other people equally with love. (Srimad Bhagavatam 11.29.15). Actions should be done with intention of giving happiness to others and to yourself. So, give happiness to others. At least don't think bad of other and don't do bad to others. There is no better dharma than helping others and there is no worse sin than giving pain to others. Gita gives importance to intention than to action itself. Actions are supposed to be with the intention of giving happiness to others seeing them as part of Paramatma. These actions will liberate you i.e. give you knowledge to realize the Absolute Truth. Ultimately, the acceptance of presence of Paramatma in everyone fills you with love towards others and your naturally want to help others and give happiness to them.
What if you cannot act with above acceptance? Give most importance to not hurting others. If you can't give happiness to people around you, the least you should try to do is to not hurt others. Try your best with the understanding that ultimately you are always pure, eternal and part of Paramatma.