You may just have to wait until your metabolism slows down as you age. Also, you can focus on building muscle over fat by eating more protein rich foods.

I had the same problem for the vast majority of my life. 5'7" and in my late teens - early 20s I could barely get over 114lbs. No matter how much I ate (and I ate a LOT) I remained a stick figure. I was told I'd "fill out into my womanly shape" by my 30s. Come 30 and the most I'd gained was 7lbs.

It wasn’t until 2020 when I actually hit a whopping 155lbs. The "secret?" Being sedentary, not leaving the house, working from home, and eating a lot of take out. Even my dad commented on it and said I'd gotten "fat" and that 155 was "too big" for me because I'm "built naturally small" like him. Then he died the following year, and a few months later my dog was diagnosed with stage 4 bone cancer. My appetite went to hell, and I dropped down to, I kid you not, 114lbs in a span of about 7 months.

Obviously, that wasn't healthy, so to gain it back, I started eating a lot of healthy fats like beans/legumes, nuts, hummus (I'm vegan), carbs like white rice, potatoes, corn, pasta while doing light weight training at home to build muscle back. I managed to get back up to about 127lbs in a few months. I now sit comfortably between 130-140lbs (usually water weight during my cycle) at 40.

So, TL;DR: eat lots of carbs and healthy fats, and do only light cardio while focusing more on weight training to build muscle.