When I quit pot the first time I tried to fill the void with alcohol...BAD BAD idea.
The second time (about 10 years later) I quit everything, and like AucklandAddict I tried to focus on healing the void not merely filling it.
For me service work - helping others, volunteering - helped give me a purpose.
It wasn't instant fulfillment - it took a little while- but it really helped me and my 'void'
I know thats not an option for everyone, but I do think finding something with a sense of purpose to it, something that adds to us rather than subtracts, is important.
Some find that in prayer, some in exercise, some in taking up a cause, some in exercise.
I'm wishing you well in finding your healing purpose WT