You are in a tough situation. I so very much feel for you, although, I can't begin to understand how hard it has probably been for you. I was lucky; my parents decided long ago (upon divorcing; I was four) to never speak ill of the other. I'm sure it was hard on them at times because people are people and love is love and breaking up is hard to do. But, they never did speak ill of one another. The entire time I grew up. So, for me, to try to put myself in your place; in a situation where I'm sure that hasn't been true, well, it's hard for me.
So, I have to go to what I do know. Which is this. Your dad loves your more than anything. It breaks his heart that he cannot see you more than he does. He wants nothing but the very most best for you. He is willing to spend as much time as you need and want talking about whatever. He WILL do what is right by you, to the best of his ability and to the best of what is fair and reasonable. It is most unfortunate that your mom will not talk to him about important things regarding your life and your future. But, no one can change that, except your mom.
I know, from what he's told me, that the two of you had a very special bond when you were younger. I know that things might be confusing to you now about why he left your mom and all; but, you must know he did not, could not, would not, ever, ever, ever, leave you. He has not. He thinks about you every single day.
You are growing into a young woman now. Lots of things have changed since you were little but certainly what has not changed is that both your mom and dad love you very much. And, now, you have a step-mother who loves you as well. Not to mention your B grandparents, aunt, uncle and cousins and your new step-grandparents, aunts, uncles and cousins.
My request of you, to a smart young lady, is to think about how to include all of these folks in your life; not to the exclusion of your mom's family, rather, as an inclusion.
In this world, sweetheart, we all need as many people as possible in our lives to be there for us, love us, and watch our backs.