How to deal with job situation?

I work for the govt. I made a mistake and accepted prescription meds from another employee. There are e-mails doenting this. I was called in by management to explain and I confessed that yes I did accept the meds out of desperation but didn't buy or sell anything. I have a feeling I will be terminated so I'm thinking maybe I should quit first so it's not on my record. What is the best way to handle this? i wish I could go back and not done this but there's nothing I can do but take responsibility and address my substance abuse issue. Please help.