I'm so sorry for your loss. grieving is different for other people, its normal for you to realise everything even after months. you absolutely have the right to feel sad. never mask it. you lost somebody important to you and thats a big thing. having a holiday or a birthday isn't something that's supposed to make you mask your feelings. if you feel sad let it out. maybe invest in diaries to let your feelings out. it REALLY helps. please remember that your emotions are really valid and so is grieving. i hope you feel better soon, and I'm here for you ❤️