I forgot my son's birthday

Tomorrow is the 2 year anniversary of our loss. William was stillborn at 31 weeks. I am bawling hysterically right now because it just dawned on me. I mean I've been thinking about him all month knowing it was coming up but .....I forgot..... my husband gently pointed it out because I made plans with my oldest and his friends mom to take the boys to skyzone tomorrow to reward him for doing so well in school. Hubby asked me when I thought would be a good time to go to the cemetery and I lost it. It completely hadn't crossed my mind and now I'm supposed to go to skyzone! I feel like a horrible mother because I forgot about him and have no idea how I'm supposed to get through tomorrow. I was going to take my oldest out to eat and to the movies too (since I owe him from forever ago). He deserves a day for him and I want to show him a good time because he deserves it but I can't imagine getting through the day in one peice. HOW could I forget!!! I'm the worst mother in history. My heart aches without him but now that we are 2 years out my life is rolling ahead and I go longer and longer periods without even a thought if him and when I sit and reflect on it I feel aweful. Losing him was the single most devastating experience if my life. I will always remember my labor with him in vivid detail. The pain you go through with childbirth is so worth it when you know you will finally get to meet that baby you are going to take home with you. You instantly forget the pain once you have them in your arms. But with him all I had to look forward to was pain. It felt like I was in labor forever, it was very quiet. And when he was finally placed in my arms the heartache I felt can not be compared to anything I've felt before.........now 2 years later I space out his birthday! How can a mother who loves and grieves her son forget his birthday. I'm just having a hard time forgiving my self and hating the predicament I find myself in. I just invited this mom and her son out 5 hrs ago. I barely know know her but am trying to build a relationship because she lives bear us and her son is in my son's kindergarten class. I feel like I can't cancel on her and it's not just her I feel like I can't cancel on my son who deserves this and is looking forward to it. I also planned all this starting later morning to early afternoon because a storm is brewing that evening. When am I going to the cemetery I couldn't even answer my husband. I feel like a failure as a mom and I feel like I somehow love William less because I couldn't even remember his birthday. Need advice please!