The question is should we eat animal protein? The fat is a different story.
Then the question comes, are humans supposed to eat meat?
Check out Dr. Milton Mills's study on the net. It is profound. There are millions of people in India who live without eating any meat at all and have never used any medication at all.
As for your health declining after 2 years. One of the issues we have here in the western world is that we do not have a history of eating vegetarian food. There is state called Gujarat in India in which everyone is a vegetarian, it is changing though because McDonald's has arrived there as well. All these people have knowledge of eating passed on by their forefathers who were vegetarians for last 5000 years plus.
I have been on raw foods for seven weeks now and always have been a vegetarian, never had meat at all. Do I have my share of illnesses, yes, I suffer from reflux, but I blame that on eating out and not being cautious and then the doctors who just give you pills without analyzing anything at all. I was given antacids which made my reflux go wild. I am better after I started raw. Other than that I do catch colds and flus or have headaches so I feel blessed that I was raised a vegetarian.
Also read a book by Dr. Balentine, "Transitioning to Vegetarianism".
If you cannot live without meats, just eat lean meats and not red meats. Humans are not equipped to kill their pray, lions and tigers are.