126 lbs. at 5'9" gives you a BMI of 18.6, which is within the healthy range, so first bit of advice: don't sweat it. Stress can have a major impact on the body's ability to maintain a healthy weight, making it more difficult to gain lean muscle and lose unwanted fat. As far as your fiancee goes, she says she loves you for your mind. Relax, dude. No matter how 'manly' you looked, you're eventually going to get old; your mind will last a lot longer than whatever physique you might have at 20 or 30. And, to put a finer point on it, she's saying she loves you, and she wants to marry you. You know what turns women on far more than being built like Hugh Jackman or Channing Tatum or whoever the latest piece of eyecandy is? Confidence. So look your insecurities in the eye and tell them to piss right off.

On to the technical stuff. I was a trainer for a high school football team, and the first thing we'd tell people is "forget the fads." There's always some new gimmick that promises to 'build muscle fast', and it's always a boatload of crap with just enough 'science' thrown in to make it not so obvious that it's a boatload of crap. Whey hydrolysates, creatine, androstenedione, taurine... Crap, crap, and more crap. The thing to understand about molecular biology is it's all just variations on the same theme: (carbon + hydrogen + something else) swimming in water. Your body doesn't build muscle out of dietary protein. Muscle tissue, like all tissues in the body, are made up mostly of carbohydrates and water. What protein the body does need to build muscle it makes on its own by adding an amine group to a carbohydrate chain. Technically, a person could gain lean muscle on a diet of nothing but twinkies and beer, provided they exercised enough. So forget the fads; eat a healthy, well balanced diet. Eat what you like. Personally I plateaued at 115 pounds (I'm 5'4", so I've got the height issue to deal with as well) until I embraced my Sicilian roots and started piling my plates high with pasta bathed in olive oil. Now, at 35, I weigh about 150 (technically just outside of my healthy weight range, but I have a 3% body fat content). I'm a contractor, so I get a lot of physical activity at work, but I haven't been in a gym since my boxing days... 15 or so years ago.

Trust me; base your diet around a mix of simple and complex carbs, and you'll easily be able to get in that 4500 calories a day. However, I question that number. Basically, you need 20 calories per pound of target body weight. Let's say you want to get to 160. that would give you a BMI of 23.6, which is right about where most guys are happy and still within the healthy range. That requires 3200 calories. You'll want more based on your activity level, but not a whole lot more. So shoot for at least 3200 and don't worry about how much you go over.

Second: you wanna get big, you gotta lift big. This isn't something you can jump right into, so I'm hesitant to go into any detail without knowing more about your current fitness level. How many times a week do you work out and what's your workout like? What's your resting heart rate? Do you have any medical issues that would keep you from lifting weights? Do you smoke, have asthma, or any other breathing difficulties? How old are you? When was your last physical? Do you have cavities? (The last question seems odd, I know, but oral health is often a good indicator of cardiac health.)