I can actually really relate to this. I also refer to my dad as my best friend and once even had a panic attack about him dying, so much so, that I ran into his room at night crying saying I didn't want him to ever die and I don't know how I'm going to function when he does. I realize this is also unhealthy and to make matters worse, my dad is really old and has a lot of health issues that make him a ticking time bomb. As much as it hurts, it helps to gradually think about your parent's eventual death before it happens so that you can better prepare yourself for it emotionally. Not thinking about it will make the actual event feel like you've been hit by a train you didn't see coming.
Realistically think about your actions, your support system, and your coping mechanisms that you will use when that actually happens. Little by little, start to depend more on yourself rather than an outside source. Try to move the feelings of safety that you have for your dad towards yourself and your own capabilities and your own strength and safety.
That's the best and only advice I can give to you because I'm in the same situation. I'm just praying I don't lose my mind when it actually happens and go into a catatonic state. Wow that'll be fun *sarcasm*.