Welcome to SR. You had a week of sobriety which you say was amazing and productive. You can have the same again and build on it this time. I think you know went wrong as you pointed out “that first drink” led you to now feel anxious and miserable. As the saying goes...if you don't pick up the first drink you cant get drunk.
Whatever you were doing that was working to keep you sober during that week i would continue to do it again and add in extra support and maybe a plan to keep you on track this time. If i were you i would also look at the circumstances surrounding this slip. Did something trigger you and make you want to drink?.
If you know what triggered you and why you picked up a drink you will hopefully be able to avoid this happening again. It is possible to be free of this addiction if you work at your recovery. There are plenty of great examples of people who have beaten it on here. You will find lots of great support and advice here. I hope you feel better, wishing you the best.