Yes. It is that simple.
I think is only that simple for some people.
I was in fairly good health until three years ago, then lost all ambition to do anything physical. Just now starting to feel like exercising again.
You sound like me. I weighed exactly the same 128 at 5'5" for over 20 years. I got depressed about a few things and went down to 108, which was really not good for me (big bones). I looked like a skeleton. Life evened itself out and I gained the weight back. But then, life took another odd turn and I started gaining weight for the 1st time in my life. Only 10 pounds, but it went directly to my mid-section. I can't take it anymore. I can't wear any of the clothes in my closet until I get back to my normal weight.