I am failing

I was doing great since September. I quit smoking marijuana (very heavy habit for nearly 20 years) and drinking at the same time. I didn't have a big problem with booze but I figured if I was quitting smoking it would be easier if I quit both. It was very tough for the first two months, tougher than I thought it would be. I withstood a lot of triggers, staying strong. I was very proud of myself. But now I have been in nursing school the past four weeks, so stressed out, while enduring a very toxic work environment that I finally ended on Wednesday...it ended very badly with the owners calling me on the phone and threatening me. On the day I put my notice in, I had a couple sips (dumped the rest out) of whiskey and smoked a bowl for first time since September. Today after being yelled at on the phone I popped open a seltzer and slammed that thing down. I feel better but I wish I'd just had the mental fortitude to have not done that. I feel like I've opened the door back up to temptation and undoing all my progress. Any advice? I want to stay sober but I feel like I'm losing the battle.