Wow... This is huge. Forgetting a wedding you helped brainstorm ideas for? I can understand why he would have been hurt. The truth is saying you forgot, while being honest might not make the situation better. Instead it might make them feel like you don't value them enough to remember. Because for some people, as long as you're close to them, the only valid excuse you'd have not to attend their wedding is either to be incapacitated, in the hospital, or to be arrested. Anything else would likely ruin the friendship.
But then, people are different.😎
It was such an embarrassing situation!!! The worst part is that, since it was a more private wedding, there was nothing on social media to alert me. On the bright side, what helped a bit in this situation is that he knew my memory isn't great, but even so, he was really upset, and rightfully so.