["Last week, I can't believe I forgot it was my best friend's birthday. I felt like I betrayed our friendship.", 'Oh no! I did that last month. I still feel bad. Get them a gift to say sorry.', 'Definitely. I tried to make it up by shooting him a text message but I still feel bad.']
Give him a call too. It is a bit more personal.
I forgot about my best friend's birthday last week and I felt like I betrayed him by forgetting about our long friendship.