Unfortunately, happiness cannot be forced, all of life is transient and happiness will come and go.Best thing you can do, and it sucks and I wish I had better advice, is try and change what you can in your life and your environment to try and improve general happiness.
This can take many forms, be it therapy, CBT and talking therapies especially, or small things, like taking half an hour to visit somewhere old or new, adding a new decoration to your room or working environment, wearing certain pieces of clothing or accessories that make you feel better.
This is of course easier said than done, and I wish there was an easier solution but there aren't any easy solution.
Just stick at it, and I promise happiness will find you)