Holly knew very deeply and very strongly in her heart how much you loved her. What happened could so easily have happened to any one of us. Release the guilt and just let your love for Holly stay with you. That is all she would want.
I am so very sorry for your loss. You did not let Cake down, please don't think that. You gave her love, just as you promised.
Run free, Cake. Godspeed, Gussie.