I kind of have this issue with my mom, I told her several years ago and brought it up recently and she told me that she thought I was doing it as a trend, but I could tell by her face this time that she doesn't believe me, so I've stopped trying.

I applaud you for continuing to try. I came out to my mom as a teenage and she kind of gave me a "yeah, ok, whatever" and we never spoke of it again. The few times I've hinted or alluded to my being attracted to women, she just outright ignores the comment. I've decided that it's not really worth my stress - if I meet a woman and have a relationship with her and I get to a place where I would want her to meet my family, I will just deal with it at that point. I understand that I may be avoiding a needed conversation - but I almost wonder if it would be easier to talk about once there is actually a person to talk about.
As far as your granddad goes, that is likely going to be a harder struggle - older generations have a harder time with this sort of thing. The concept that sexuality is fluid is super new and a lot of people can't wrap their head around it. You're gay or straight. That's it. Patience is key.