Hi, 

I lost my dad to the same in March 2019, his first grandchild came along 2 weeks after we buried him. 
I am the same, my son has been the most precious gift and an amazing distraction for everyone, but like you I have put all my energy in to him. 
I do think about my dad everyday, but the days have just passed so quickly. Some times it hits me like a tonne of bricks out of no where and I hysterically cry, is that grieving? I just don't know? 
Have you seen a counsellor? I haven't as I still maintain a counsellor is trained and will help you through, but how could they possibly understand unless they have felt this grief alongside other factors like having a baby so close?