Hi lozza, it sounds like you were very close to your grandparents. And it's so easy to start thinking of all the things you "could have", "should have done" at a time like this but at the time you didn't have the hindsight you have now. You just did what you could at the time.
So please don't regret what happened in the past, there is nothing you can do about that now, and the love you shared with them was the strongest thing in the relationship. It doesn't deserve to be overshadowed by all the "What if........"'s, ""Maybe.........."'s. So let it have it's rightful place, hey? First and foremost
You clearly mattered a lot to them, so don't let that go.
And cherish some of that closeness, the meaningful, the positive, the important times you had with them. All those feelings/times are still there for you to draw on, to give you comfort.
And as for your mum........"she is scared I will regret it if I don't see her and say goodbye".........the decision is entirely yours, what's right for you. And if you decide/feel that you can't go, well if you "bowed to the pressure" and went for the sake of someone else then there's every chance you'd regret going much more than if you hadn't have gone.
So if you don't go, maybe you can think of a way to say "goodbye" to her in your own way?? Even talk to your mum about how you may to do that if you want, because she's probably/hopefully thinking of what might make things easier for you right now.
I'm just thinking, maybe there's a place that was special to her in her life, maybe you could leave a note/something there for her............but just something personal??
But really well done on improving with the drinking, it couldn't have been easy so kudos to you!! It might just be that you need a little extra support with that right now, which would be completely understandable, so do you think you could do some AA meetings or even check out the Addictions forum on here for support??
But here if you want to talk.............

Alison