I read you and I understand that pain, when you celebrate your first birthday with the absence of a loved one, so I lived my 27th birthday with the absence of my grandmother, where I missed so much those cakes that she made that were unique.
It has only been 4 months, it is too soon to get over it, grief should be carried as the heart and feelings command, repressing is worse but I agree with your mom, life should also be celebrated and I am sure your dad would have wanted to be reunited on your birthday.
I don't know if I can wish you happy birthday but I can send you my strength because yes, the first year is the hardest but as someone once told me: "let's remember those who are gone and celebrate those of us who are still here".
Wise words dear friend. Especially now I have my mom who needs all the love in the world too. I thought I knew bereavement because of the amount of loved ones I have lost over time, but when it is your real nuclear family, as in our cases, is where we really know this feeling. I thought I was doing pretty well, but I didn't remember the important dates that hit the hardest. Thank you very much for reading me, for your support and for your good wishes for me...