Welcome, Jules! Oh how many times I've done the same thing. Felt so proud of my sobriety and allowed the alcoholic in me to convince me that I was fine to have just one. I'm the same with drinking - one flips the switch and I cannot focus on anything but getting more, and more is never enough. Ever.
You've done well to come in here and confess! Today is a brand new day. You know what you need to do, so do it. the obsession with which we are afflicted is activated not even by one drink, but one sip... And the cycle is set in motion yet again. I despise that cycle. When you're in it, it feels like there is no way out except down further and further. But you've come out - and now is your chance to put the past in the past and focus on taking action today to prevent further relapse. It's great that you are attending meetings. If you don't have a sponsor, see if you can get one. That helped me in staying accountable and learning about this disease. Keep your head up!! It's never too late :-)