Alright, I’ll give you guys a little history so you can kinda see where I’m coming from. Basically, I have a metabolism that would rival a bullet train in speed. I’ve always had it. I could literally eat everything and anything growing up and never gain any weight. At 5’5, I only weighed about 105 pounds before I went into the military. When I came out (discharged from basic for stress fractures), I weighed about 135 and most of it was muscle, so that took care of my metabolism being a pest quite nicely.
Unfortunately, I don’t have the ability to eat 3 square meals a day that are designed solely to energize me anymore and it’s quite impractical to smoke myself throughout the day to simulate a basic training environment. I have started working out again, but since I just started, I’m going easy so I don’t kill myself. Problem is, I weigh 123 right now. I’m sure this is a problem that many people would love to have but I don’t see much difference between chronically overweight and chronically underweight; both have health concerns and they’re both irritating as hell.
I just started using mypyramidtracker.gov which basically just tracks your food intake and compares it to the food pyramid and crunches numbers for you…today is a good example. I ate a /shitload/ of food (although most of it was nuts, fruit, etc. healthy food) and came in at 1700 calories when I should be consuming 2200 calories. I wasn’t really hungry at all the entire day. I sure as hell was hungry constantly in basic though and I’m aware that caffeine has the damnable ability to stifle appetite.
So basically my question is what measures should I take to try to keep my weight at a healthy range? Should I just try to do as much strength training as possible to get back muscle mass since muscle weighs more than fat? Should I cut out all caffeine products so they stop screwing with my metabolism and appetite? Any advice will be appreciated!
Edited to add: If it makes a difference, only soda I drink is diet pepsi so there’s no negative property of the soda I drink such as lots of calories or fat, just caffeine. Well, and the fact that an acid in soda is used as a rust remover so I’m sure that’s not helping either. 
If you drink any kind of protein shakes then add flaxseed oil to them. Increase your intake of healthy fats and the calories will go up. If you have an avoidance to fat then not sure what to recommend to ya.
Stay away from caffeine. Anything that’ll keep you from eating should be avoided.
If you want muscle then perform aesthetic training. Strength training while great is not synonymous to muscle building. Increase protein and fat intake. Eat every 2-3 hours.
I agree that adding fat will add calories, however, I would consider your cholesterol levels first and/or if you have a family history of heart disease (of course you can eat healthy mono-saturated fats). At 5’5" and 123 you might be slightly underweight but not badly. If you are not eating much and not hungry (you lucky dog you!!) then it doesn’t make sense to me to stuff a bunch of extra food in your mouth just to make a calorie count. If there is any way to make them more calorie dense than that is a good option. If not protein shakes, try the canned/bottled nutrition drinks like Boost which gives you a good range of nutrients. I would definitely skip the caffeinated beverages with such a small appetite. They are just filling you up, likely. Try eating without drinking until after the meal as well. Damn, man, I WISH I had the problem of being underweight rather than overweight!!