First of all , you deserve a big round of applause for 3 and Half weeks sobriety. That , too, after 8 years.
What happened to you is not rare at all. It had happened to many of us. We have been there and done that.
Now, for a moment, forget any thing about the relapse. If you look at more closely, you not only overcame withdrawal symptoms but started enjoying the benefits of sober life. Your motivation reached new heights. You started doing may things because your energy levels went up , both physically as well as mentally. Yes, you might have over committed and demanded more from yourselves. But , nothing is wrong with that . And I fell , that might have not caused the relapse. You will be the best judge . However, from our experience, this happens due to complacency which sets in, once our life becomes better and better while being sober. Then, slowly addictive voice creeps in. It says " See the life is much better now. I could quit for many weeks and I will be able to do it , as and when I desire again.. Hence, no harm in just enjoying one drink only. I think I can handle it "" This may sound familiar to many of our SR friends..
Finally, these were just my thoughts . The best course is just to move on and focus more on the benefits you could enjoy during last three and half weeks and make those benefits available to you for the rest of the life.. All the best.