MH says if the mind is convinced the body will follow. Maybe you need to study more about vegetarianism, especially about protein, and what meat does to your body when you eat it.
Then go slowly instead of all at once. Maybe only eat fish and chicken for a while and give up red meat. Then only eat meat twice a week or something. Then only eat fish and then only once a week.
When you tell your mind you are qutting something never to indulge again it freaks out and goes into a panic, which is why you crave whatever it is you're trying to give up. Sometimes you have to trick your mind.