Calorie counting.

Simply being vegetarian and not eating sweets isn’t enough if you aren’t actually tracking your food. You can eat “healthy” and still eat way too much, and that will cause you to gain weight.

Plug your stats into my fitness pal. And follow what it tells you. Measuring/weighing is especially helpful when you’re first starting out.

There is nothing wrong with nuts. They’re good for you. That being said, they are calorie-dense which is why you need to measure and track. If you are eyeballing it and you think you are eating one ounce, but you actually ate 3 ounces, that’s a difference of about 320 calories. Nuts are easy to mindlessly snack on, so be mindful! (As a rough estimate, 1 ounce of nuts = about 28 peanuts, or 17 cashews, or 22 almonds. Which really isn’t much at all.)

Also if you are strictly vegetarian not vegan, dairy and eggs are great sources of protein! Before I had to cut dairy, my favorites were Light & Fit greek yogurt (80 calories, 12g protein) and Cheese Heads light string cheese (50 calories, 7g protein).

I have the same stats as you (but am in my twenties), but have you always stored fat in your midriff? I feel like most people will have one area that clings on to fat at all cost (mine is my thighs personally), but I might ask your doctor if you've randomly started to accumulate fat in a new location.

I'm also a vegetation, and nuts have a suuuper high calorie to volume ratio. For protein I eat Greek yogurt, eggs, beans, tofu, tempeh, lentils, cheese, edamame, mushrooms, spinach, and mung beans to name a few. I think all of these have higher protein to calorie ratios and lower calorie to volume ratios, so they might be helpful if you're trying to cut calories and keep up your protein intake. I definitely agree with @bribucks that you should count calories and measure portions (even if you don't do it long term) for at least a week to get a more accurate idea of how many calories you consume per day. Estimating is often very misleading.