Listen, 100 pound weight gain is NOT healthy. You know this. And yes, it is very common for many women in their early to mid 30's to experience a change in their metabolism, and start gaining weight. I believe you that its genetic.

That's no reason to give up.

I am in the same boat as you, I put on an extra 40 pounds, very slowly, but it happened. And I am unhappy. I am now counting calories, and slowly loosing the weight. It's simple math. Google a calorie calculator. It will show you how many calories it takes to maintain at your current weight, and how many it takes to loose it. It really is that simple.

Fine you haven't changed your eating habits, and are gaining crazy weight. That just means your body can no longer burn those extra calories without you exorcising, or moving more. That's all it means. It doesn't mean you can't loose the weight. It just means it is going to take more effort for you than before.