Your metabolism only slows a marginal amount as you age. It isn't as drastic as the Dr. Oz culture of "blame it on my metabolism" has led you to believe.

Track. Your. Nutrition. Period.

Before even clicking on this, a thought popped into my head, which was "I bet he's going to say he's eating "fine", but not quantify that with how much". Fact is it's easy to overeat. Even if you aren't scarfing cheeseburgers.

MyFitnessPal, as suggested, is a great tool. Track your food, AND it'll help calculate a healthy caloric intake for you. I've been using that for all of these nearly 120 pounds I've lost. Hope I'm not sounding too harsh by the way; it's just the essential. Diet is 90% of it, exercise is only 10%. Years ago you lost weight just by adding exercise which is great! But if it isn't working, it's time to start RECORDING nutrition! You'd be amazed how calories can sneak up on you.