Too numb to care? Oh hon, get a grip.....It's not like he didn't do anything for your birthday.
When hubby or I forget each other's birthday, we get a sense of humour and tease the other one by singing ourselves the birthday song. Then the forgetee laughs and remembers the gift and all is fixed. It's far more productive than getting in a huff over the whole thing.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.