When I became vegetarian several years ago, I did experience a boost in energy and mental clarity. But I did it simultaneously with meditation, special physical excersises and so on. Some time ago I went to a seminar on traditional Slavic personal growth techniques.
It was rather anti-new age. The guy who was leading the seminar said that the personal development practices should differ depending on the land and culture you are in. Something that works in Eastern Asia and India, might not work in Russia. He said that vegetarian diet is not efficient enough in Russian climate, especially in winter. It was against everything I knew up to that point, but decided to approach it with open mind and do a 30-day non-vegetarian test.
The idea seemed like going back to the bear cave, but, why not, if it is just for 30 days. So I did it and guess what? Nothing happened. My energy levels did not decrease, my mind clarity didn't decrease. It was surprising, because I expected it to happen. After some thinking I've got the idea that maybe it was not the meat that affected me in the early years, but the amount of food. In recent years I eat much, much less then before.
I did not become active meat eater at the end of experiment, but for now, I am not a strict vegetarian and allow myself a chicken or a meat dish once in a while. Junk food still is out of the question.
So, Pavel, there is no universal answer to this, follow your feeling.
Also, remember, that in our country the quality of food may not be very good - in Moscow most of it is terrible. And with vegetarian diet you have to aim for the best, which may be expensive. Otherwise, you run the risk of not getting enough nutrients that should be there, but are not. Try compensating with vitamins and minerals. Especially in autumn, winter and early spring, when we don't get enough of nutrients. Try iodium, drink a lot of clean water, may be you have some cravings for something other then meat. Follow them, listen to your body, may be it is missing something.
Hope this helps.