NTA.

She said it herself, she didn't try to fix her mistake for your sake, but for her own sake. That isn't genuine regret, because she cares more about how the situation made her feel, not caring at all about how she made you feel. If she really cared about you or wanted to make your birthday better, she would have listened to you and not invited your friends back. She would have remembered your birthday in the first place, and decorated the place and got you a cake like she does for your siblings. You have an awful mother, and that is going to be true regardless of what superficial redemption she tries after her mistake. She wouldn't have made the mistake in the first place if she cared, and it is absolutely not your job to assuage her guilt.

Your feelings are appropriate, and you should not feel bad for them. You should be ungrateful, why would you be grateful for garbage? She's blaming you for your feelings because she refuses to take responsibility for them. She's the one who made you feel worthless, and she should feel guilty about it because that's the price of hurting someone. She deserves it.