Excessive Grief

Understanding the lesson in everything happens for a reason and trying to make sense of profound grief. I volunteer with a small animal rescue and it is my greatest life passion. I could not imagine not doing it despite the hard work and sadness involved. The moments of joy outweigh the harder times and I truly love the work I do with the animals.
Recently my nightmare of all nightmares happened. I have always sworn that I would never intentionally or purposefully hurt an animal. I struggle to admit that this is not the case anymore as a few weeks ago while I was cleaning out the exercise pen, one of the rabbits got under my foot as I stepped down and I ended up stepping on her. It all happened so fast and I was physically sick as I picked her up and held her as she passed from this world.
I told her that I loved her, I was soooo sorry and to please pass on so she would not suffer. She did pass before I could even get to the car with her.
It has been a few weeks now and I cannot seem to get past the guilt, the GRIEF and the pain. I was supposed to be there for her and I hurt her. I cannot stop the images that flood into my head each day and if there was any way possible to undo what happened I would do anything to make it so.
I know that there is a lesson to learn from all tragedy and things happen for a reason but I struggle with this one.
I have searched the net and asked my closest friends but I still am unable to find any meaning in this.
I pray that you can help me with some direction as the grief and guilt are making each day a struggle. I don’t know where to turn. Please help.