Hi Alberta! Congratulations on the weight loss and maintenance.
Boy do I hear you on how hard it is to lose those last few pounds! I can’t seem to pry one more lousy pound off my body now regardless of how clean I eat, how carefully I control my calories (like you, I'm an obsessive Fitday user), and how much I exercise. It can be SO frustrating!
Some thoughts — you’re pretty small already at 138 and 5’ 6” — that’s about what I weigh and I’m two inches shorter. Do you really think you have weight left to lose? Or could you already be at the right weight for you? Have you had your body fat % checked? — that might tell you if you’re at the right weight or still have fat left to lose.
As for how many calories to eat, I’ve always heard not to drop under 1200 calories a day — how many are you eating now? For most of us, it’s a trial and error process to discover what’s maintenance and what lets us lose fat. I’ve found that 1400—1500 is maintenance for me, unfortunately, and it’s hard for me to go lower without being hungry all the time, so I’m stuck where I am.
My experience with metabolic testing is that it’s interesting but not very accurate, at least in my case. I did the BodyGem test twice (about nine months apart) — the first time I got a RMR of 1490 and the second came in at 1600. Now that’s supposed to be the number of calories that I’d burn if I laid in bed all day, without even considering normal activities and exercise. I do at least 45 minutes of cardio every day and lift weights five days a week, so theoretically I should be able to eat over 2000 calories/day and maintain my weight. In reality, if I eat over 1600 calories, I gain. Mel and Jiffypop here at LWL have also had the BodyGem testing done and both got results way higher than their own personal experiences with weight loss and maintenance. Same thing for me with mathematical calculations. I WISH that I could eat that much and maintain, but I know from experience that I can’t. So

— I think it comes down to experimenting, tracking, and seeing what works for your unique body.