Sorry for the long post but I know a LOT about this subject.
I was a complete vegetarian for 8 years. I didn't even eat fish, which some self-proclaimed vegs do. I managed my diet really well, I ate all the right things. Couldn't stand soy though or tofu so I never ate that crap. I ended up needing some supplements. Most people who are veg and don't supplement start to get weak and show deficiency signs. The problem with a vegetarian diet in the modern world is that most vegetables are now grown on depleted soil so they are lacking in major nutrition, so you end up having to supplement a lot. This would be true regardless if you ate meat or not, but meat gives you a lot of the B vitamins, iron, protein (especially the sulfur containing amino acids methionine and cysteine which veggies tend to lack), vitamin A, etc. the list goes on. If you ate a steak every few weeks or liver every few months, you'd stock up on these, but as a veg you need constant supplying. Given the state of modern food it's hard, and to be honest it requires money or access to good farm food. Even then, there are so many marketing gimmicks that it's hard to know what is genuinely nutritious sometimes.
I eventually moved to China (long story) and while I was there meat was unavoidable where I was. People had no concept of vegetarianism. The default word for meat in china (肉 rou) means pork. So when I said I didn't eat meat (rou), they thought I meant pork so they brought me other meat. Then I'd have to explain that I don't eat ANIMALS, and they just didn't comprehend. So, long story short, I started eating meat again, and HOLY COW my energy levels went way up and I felt a lot healthier again!! I decided then that meat should obviously be part of my diet, but in the beginning I thought more was better so I started having meat every day until I started to feel heavy, bloated, and disgusting.
I'm an alternative health practitioner now with the title of Doctor in my region, and I've come to understand that everyone's constitution is different. I need animal protein in my diet. I don't need it every day, I just need a bit every few days and I'm good to go - but I can't live without it. Some people need a bit of it daily, others virtually never need it. My mother almost never eats meat and her iron and hemoglobin levels are normal. My father, sister and I share the same constitution in that we need meat, but my sister is a stubborn vegetarian and she is incredibly unhealthy.
Vegetarianism, as a political movement, and ominvorism as a political movement, are both fraught with propaganda. Most people in North America can do with less meat. If you're eating it every meal then something is not right. In the wild, animal protein was a rare commodity for the hunter gatherers. Humans ate meat but mostly subsisted off of vegetation, with animal protein supplemented. Of course, in the more tropical climates where vegetation is abundant, people can avoid meat no problem. In my opinion, if you live in a cold northern climate, most people need meat, and it needs to be served warm.
Another important thing I've learned is that people eat way too much cold food in the western world. Your body doesn't absorb food properly when it's right from the fridge. It doesn't undergo complete fermentation because once that cold food hits your gut, the blood vessels contract and digestion slows down. Muscle contraction moves the food along and warms it up but it comes out the other end as loose stool or even diarrhea, which means incomplete absorption. Whatever it is you eat, make sure it's warm, unless you live in a climate that's an inferno then usually some cold food is okay. If you're a veg and eat a lot of raw food, you can warm it up by adding warming spices like ginger, garlic, etc. Chemical warming still produces heat exothermically which helps digestion. If you eat cold food then your body expends energy to warm it up and this leads to digestive deficiency, and you lose nutrients. In places like Asia, eating cold food like ice cream is a relatively new thing and most of the old people frown upon it. Virtually all food there is cooked except in the really hot weather when they serve cold and vinegared vegetables. But now, because they have gotten ideas from the west, a lot more people have digestive deficiencies from eating refrigerated food.
I could go on all day about this. The digestion system is my specialty in medicine. Short story is... there is no one-size-fits-all solution to diet. You have to eat according to your own constitution and not what someone else tells you to eat, and this takes experimentation and self-awareness. If you're eating something just because it makes you feel good (like a lot of meat eaters) then you aren't eating according to your constitution but are emotional-eating. Likewise, if you feel like you are suffering through vegetarianism then you're either not doing it right or you shouldn't be doing it at all.