I'm sorry, grief is the worst.

To attack the sadness directly, which is a result of chemicals in your brain, there are specific other chemicals you can add that will raise serotonin and norepinephrine and also dopamine. It's not the most sustainable solution, however. Other ways of boosting those neurotransmitters include running real hard, getting a tattoo, having sex.

Processing the emotions, possibly with the help of a professional, is the recommended long term solution though. It won't bring them back, but it'll help understand the pain, and hopefully heal it.