First of all Im so sorry that you are pain, missing someone & just not feeling the happiest, I really feel with you & hope Hashem sends you comfort soon.
I think there are different types of joy,- some which may be still possible for you to feel – the feeling of simcha of being Hashem’s Am Hanivchar, we have a purpose in life (look at the goyims lack of direction and purpose &how difficult their lives are), the simcha of being cleansed from YK and been given a new opportunity to serve Hashem with a clean slate, the joy of knowing that (especially when we are in the Succah) we are under Hashems protection… He has a plan for us, He knows what we are experiencing and He has a reason for it… if we really internalise these things I think it can give a person strength and hopefully some JOY 🙂
All the best to you, good luck 🙂