Crikey we are both going through it today! (see my other threads) Really sorry to hear this, you have had a lot to deal with the past few weeks.
One thing I will say is, any fish in your house (and there are many!) are very lucky! You always do your absolute best for your animals and you have no reason to beat your self up with it! If they are problem behavior wise you do your best to separate them but if thats not possible then its just like in the wild, if they bite off more than they can handle nature takes over and there is little we can do to prevent that.
I always think the tank is the hobby and the fish are pets. And there are truly heartbreaking moments in this "hobby", but seeing the pets thriving every day because of the time and investment to learning about the hobby and making sure you are doing the right thing helps the balance for me. Though when things are as raw as they are for us both right now I can fully imagine selling up!
Wills