Breaking My Little's Rules?

I've been playing this DDLG role with a dear friend of mine who is also forty and prefers to be in the little space. It sounds silly, but it has brought us closer together and helped us understand each other better on different levels. We decided early on that we were both mature enough to handle it without crossing any lines or boundaries. Recently though, I've felt like maybe I should have taken my role more seriously than I did. A week ago, he asked me to pick up some milk for him since he was short of funds at the time. He reminded me three times about this, and I even jotted it down on a sticky note to remember better. I ended up going grocery shopping anyway but skipped the aisle where the milk was located because I saw that they were out of my preferred brand. Instead of stopping to check for another option or asking him if he'd accept something else, I just left without getting the milk at all. When we talked later and he mentioned it again, I felt terrible for forgetting such an important little task. He was understanding but pointed out that as a responsible Mommy, I should have done better, even when things got inconvenient. It's made me question how well I really fit this role sometimes.