Jane I am sorry your dad died. My husband died aged 47 when our daughter was 20 and son 16. This was in 2004. They are 39 and nearly 35. They talked to me and their friends this was in 2004 but they had lived with the knowledge their dad was not going to live 5 years since they where 17 and 13.
You need to get in touch with groups that are available to help young people cope with the death of a parent. They will help you.
What I will say is the same thing I say to all widows/ widowers don't try and be brave if you want to cry ,shout,swear, scream or hit a pillow do it don't hold your feelings in it will only hurt you more. Talk to your dad out loud I promise it helps. I have been doing that for 18 years . I talk to my husband swore,blamed him for leaving us,shouted at him but it has helped me . If you keep things bottled up it will hurt more.
Your grief is different from your mom's. I can't promise it will get easier as you get older but you learn to cope. My husband made me promise to live the best life you can. I am sure that your dad would want that for you.
You are a young woman and have a whole life ahead of you. Grief at your age is not fair but you can learn to cope it just takes time. You will always love your dad and he will always be part of you. I am an atheist what has given me comfort our children and 5 grandson's all carry part of my husband's DNA . That's why I say your dad is part of you.
Find a group of young people coping with the death of a parent you need to be with others your age. As I said your grief is different from your mom's . That's how it should be.
Take care you will learn to cope so give yourself time. Love never dies. ?