I'm not going to ask you to excuse your mom for forgetting, but it's happened to almost everyone that I've ever known at one time or another - for one reason or another, your mom was overwhelmed and the day slipped by. Then, there are those parents/siblings/friends who truly want to make every milestone a positive event and make it Their Duty to see that it is. Then...then, there are the people who remember milestones for their own purposes - not to make it a special day/event, but to create something that they can refer to in the future, "Remember all of those special times I made for you and THIS is how you repay my kindness?"
I turned 18 during my senior year in High School. To me, it was a very important milestone. I was eligible to vote. I was legally able to drink beer or wine in a college pub (yes, the legal beer/wine age was 18 way back then!). I was legally responsible for all of my decisions. I had reached the "age of consent." That birthday came and went like any other day, without a card, without a verbal recognition, without a gift, etc. I was living at home, too!
The one thing that you could do with positive energy in mind is to kindly and gently let your mom know that she let your birthday go by without recognizing it, the next time you talk to her. No need to accuse, spout angry epithets, or hold a grudge. Just let her know that it rather hurt your feelings, but you're not holding a grudge about it - just letting her know. Then, ask her if there's anything going on that's causing her undue stress that she might have forgotten. We never know what is going on in the lives and minds of our family members, friends, coworkers, etc. - we're JUST not in their shoes.
Rather than allowing your Self to become sucked down in the bottomless anger vortex, go rent "Sixteen Candles," and laugh your HEAD off.
And, it is NEVER "fun" to play "Tit for Tat" and perpetuate negativity by paying someone back for their transgression - he without sin may cast the first stone, and all of that. Let it go and have some laughs.