I can see two possible answers, but they are probably both wrong....
... and if one is correct you will probably have to explain a bit more for me to understand...
- Are you
Energy?
Everyone is part of me, even before the moment they open their eyes, doesn't
matter if they are alive or not.
We all have some potential energy, kinetic energy etc.
I can be your happiness or your misery.
lack of energy could be misery
You can leave me, but I will never leave you.
not sure - this may not fit
Time can change me, but I can also change the time.
energy can change from one form to another - and in different gravitational potentials the speed of time can change or from acceleration to high speed (e.g. twins paradox - due to relativity)
I am the most powerful drug.
exercising can be a powerful drug....
Some spend their entire lives trying to find me.
people try to get fit
I can never be consumed, I can never be destroyed.
energy can neither be created nor destroyed
I am everything and nothing,
energy is in everything, but not a particular thing on its own
So that was first attempt....
... now second attempt
- Are you
God or some spiritual being/lifeforce idea
Everyone is part of me, even before the moment they open their eyes, doesn't
matter if they are alive or not.
this could be argued spiritually
I can be your happiness or your misery.
this fits as well
You can leave me, but I will never leave you.
this fits as well
Time can change me, but I can also change the time.
first part may not fit, but second part fits
I am the most powerful drug.
spiritual experiences can be very powerful...
Some spend their entire lives trying to find me.
this fits
I can never be consumed, I can never be destroyed.
and this fits
I am everything and nothing,
not sure about this part
so to conclude.....
one scientific answer and one more 'spiritual' answer....