Okwhat, first of all, I'm sorry, sorry you lost someone so close to you and you are hurting. I know what that's like.
I found one of the hardest things to accept was acceptance itself. Actually just accepting whats happened and trying to come to terms with it. The only thing that helped me was time and even now, 4 years down the line like yourself, I really struggle with it. Sometimes I can get on with life and go with the flow, then at other times I'm just down with everything really badly.
Its hard to accept whats done is done. I too look back and think if I'd done that, been nicer, been more patient, put a bloody smile on my face, gave her a hug...its endless. All I can say to myself is I done what I could at the time, with what I was living with. 4 years on and with hindsight I'd have done things differently, but I didn't have that experience then.
I think what I'm trying to say to you is you did what you did and at the time you couldn't do more, or you felt you couldn't do more. And I'm sure she knew it and was grateful for all you did do, which I bet was a whole lot more than what you imagine.
I hope I've helped a teeny bit, its really hard to get your head around losing someone you loved so much and accepting the reality of it all. I think with all the hindsight we have now we forget to be kind to ourselves and accept we tried our best. 