I think this is why it is soooo important to observe and respect your body. There are also two issues here; ethics and nutritional needs. Unfortunately many sources out there meld the two into one. They should really be considered separately in my opinion.
I think that if you are ethically opposed to meat but your health is failing or you are craving meat then the first step is to honestly evaluate what the health issues are. Write each one down (physiological cravings are a sign of missing something nutritionally). Then I suggest you begin a careful food journal and include all that you eat and drink, supplements, meds, exercise, sleep, and how you feel. You can observe the following 7 characteristics: hunger, cravings, energy, mood, mental clarity, sleep quality, and GI function. Ideally you can go 4-6 hours without getting hungry (this depends on the person and your mental and physical activity), you will have no physiological cravings, your mood will be even-keeled, your mind will be clear and sharp, you have plenty of calm energy all day long, you sleep well at night, and you have no issues with GI function. In addition you can observe any of your specific health issues.
Once you observe what you are eating, consuming, etc and how you feel for a month or two you may begin to notice patterns. This will help you see what may be missing. For example, if you eat oatmeal and yogurt with fruit for breakfast but you always get hungry and irritable 2 hours later, then you'll know something is missing. Then the key is to experiment and try to add things. There are only 3 macronutrients so this narrows it down. Either you need more protein, fat, or carbohydrates. Most people eating a vegetarian diet do not lack carbs. So try adding more fat or protein and see what happens. Of course this can be done without using meat: add a whole avocado, or a an egg or a bowl of lentils for example.
And I have observed over and over that needs can change. For example, when people are stressed they tend to need more protein. And many women experience some shifts before and during menstruation.
The fact is that we all have different needs and some people do great on a vegetarian diet and others suffer because no matter how many legumes, eggs, etc they eat, they are still not getting enough protein and/or fat. These people will then have to choose; which is more important, their health or the lives of animals.