I did not try to go vegetarian, but out of interest I tried to expand my eating habits and I find it very hard to replace meat with an adequate source of protein.

  * Beans and lentils give me digestive problems, so I can have them maybe twice a month at most.
  * Dairy is fine, but it is not better than consuming meat at all in any respect, so what's the point? Chicken and pork are better than diary if you care about ecological impact.
  * Nuts are very expensive, I have them anyway, but I could not see them as a reliable source of protein due to cost.
  * Eggs are fine, but I can only have so much in a week before I get fed up.
  * Veggie alternatives taste and smell terrible in my experience. 
  * I liked tofu, nothing to write home about in terms of taste, but I can see myself having it once a week.
  * I also eat canned herring and wild caught salmon. Haven't checked on their ecological impact but it is a good source of protein for once a week. 

But really, there is nothing that can match the nutritional density of a piece of chicken, pork or beef. It is packed with protein and easily digestible, you can buy it at any store, you can cook it very easily in a variety of ways, and you can rest assured you have a complete meal if you have it with a side of greens and some carbs.