You can absolutely lose weight while being vegan. You do need protein, but there are vegan options: tofu, beans, tempeh, seitan. If you are eating foods high in protein, along with fresh fruits, vegetables and whole grains, you will lose weight. It's about being vegan "the right way." Because, of course, if your way of being vegan was to eat too many carbohydrates, too much sugar, too much fat, and too much salt, you would not achieve your goal of losing weight. Have you thought of meeting with a dietician who is knowledgable about vegan diets? S/he could help you create healthy vegan meal plans that would help you lose weight. This would probably help satisfy your needs and let your T know that you are on the right track.