For your height and weight, 138 pounds is completely normal, so likely all of the work that you are doing is helping to keep you there.
Here's the thing. For the average person of your height and weight, your Basil Metabolic Rate (the amount of calories you burn a day resting) is between about 1300-1500 depending on your age.
You should be happy that you are this healthy. There are plenty of people in the world who would give anything to be your size.
Now 120 pounds is still within a healthy weight range so if you are still determined to lose weight, the only way you can make this happen is you are either going to have to eat less or exercise more to make this happen.
Eating more will give you the fastests results, however, it will also be the hardest to maintain in the long run. If you choose to exercise more then it will probably take a little longer, but the results should be longer lasting.
Again, it doesn't sound like you are doing anything wrong, you just aren't doing enough to get to where you need to me. You are still healthy where you are.