i hate to break it to you, but if you're over about 24 years old, your metabolism is already slowing. 24 is right around the point where you start burning fewer and fewer calories each year as your body just stops repairing and replacing stuff. the good news is that while your metabolism is slowing, it's a tiny difference. we're talking 5-10 cals per day difference per year past that point. so while you can't help any of that, being more active is what you can do as a counterbalance, which you are already doing.
as far as 1600-1800 being too little, well, how many calories do you burn in a day? there isn't a base universal line when it comes to energy expenditure [the 1200 and 1500 minimums for women and men are nutrient minimums] because everyone is different. if you're working on vanity pounds you probably don't want to be creating a deficit greater than about 200 cals. so if your daily burn is in the 1800-1900 range, you're where you want to be. if your daily burn is 3000 cals a day, you're eating way too little. it's not a big deal if you're a few hundred calories off on the day to day, but make sure that your calorie expenditures and calorie intake are averaging pretty close together with the expenditures being just a little higher than the intake if you are trying to lose.
-google first. ask questions later.