I know what you mean OP, but I definitely love my father a lot and it would be absolutly horrible if anything happens to him, I worry about him so much. (He is in his 70's now so is a lot older than all my friends dads, thats also part of the reason why I find it so hard to get along with him) He also suffers from bipolar disorder. All my life he never really got too involved with my brother and me, and all he ever seems to do is sit in his room and only ever comes down to eat (on his own), watch the news or go out the house. It's good though, I like keeping space between us because he just explodes because of the smallest (and I mean small) of things. It got to the point where you were afraid to have any sort of conversation in case he loses it and starts shouting and throwing things. Oh and this doesn't just happen to me, my brother and mum, we had a dinner guest over once and he actually lost his temper and threw him out of our house! He's even shouted at me loudly in public, in front of my friends and I find it really traumatic and embarrassing so I just make sure I never go anywhere with him now (but before it really affected my self esteem in general I feel, I would look at my friends who had normal dads who they could actually go out with and have a proper conversation with).
Though recently he has calmed down a bit and as I said I keep talking between us to the bare minimum so our relationship is hardly great and I am quite bitter about all the times he has shown us up in public, shouted at us for no reason, or the fact that he didn't seem too thrilled to have me and my brother when we were younger. Despite all this I still love him as I said as he is the kindest and most generous person I know, I wouldn't change him for the world because everything happens for a reason, its just his disorder and age is getting in the way I feel. It's quite weird writing this down because I have never ever talked about this to anyone except for my brother and mum...