I think you just have to let yourself grieve the way you need to. I can't even imagine what you have gone through or are going through. I have lost a parent and I know the process of grieving takes time. It took quite awhile for me to not fear that I would lose other people close to me, and I feel that it would take longer for a child - as we tend to be protective of our children anyway.
I don't think that your fear for your older son is making you a worse person. It hasn't been that long since Fox's death, and you can't hurry up that process of dealing with what happened.
You seem to me that you are very aware how you have changed, and in time, you will let go of those fears. But it does take time. Stop beating yourself up for being human.
From what I know of you, I have seen a good person and definately a good parent. So I think you are doing what you need to do. You just can't hurry that process up...
Great works are performed, not by strength, but by perseverance.