I'm also (close to) 6ft tall, and have also always been quite, ah, portly.
You say thirsty all the time. How much do you drink? You need to be getting through about two litres of water a day (more on hot days or days when you exercise). If you drink more than that on cool or inactive days, see your doctor - particularly if you happen to be *ahem* itchy in the ladies' area (diabetics' tiddle has a high sugar content, so smells fairly sweet).
At your weight and age, yes, it's possible you may have developed Type 2 Diabetes. Best to get bloods done ASAP just to be sure.
No matter how the results come back, you're going to have to make a concerted effort to stop being lazy about your food. Being morbidly obese is enough in itself to make people tired all the time (hell, I'd take afternoon naps over and above my 8-9 hours each night), and the only way around it is to get up, start exercising, and force yourself to take care of yourself.
It isn't easy. But if diabetes runs in your family you're at increased risk, and you sure as hell don't want to loose a foot, your eyesight, or die in twenty years' time do you?
So, suggestions:
1: Don't be afraid to talk to the instructors at your gym. It's their job to help you. Tell them you need to lose weight and they should work with you to create a training regimen.
2: Use an online calculator to figure out how many calories your body requires a day. For example, this one:
Calorie Calculator
3: Eat fewer calories than this per day. Don't go crazy and try to get by on an apple a day. Just shoot for 500 fewer per day until you're used to eating less, then you can vary it (I aim for about 800 fewer per day). This leads to a nice sustainable weight loss.
4: Be prepared for the fact that there
is no quick solution. You will be eating less than you require for about two years to reach your target weight, and you will regularly have to re-calculate your calorific requirements as your weight comes off and your fitness level increases. I suggest re-doing the calculator every two months.
5: Never weigh yourself more than once a week, but do keep track of your weight loss. Do your best never to skip a weigh-in, as it keeps you focussed and accountable.
6: Cut out refined cane sugar. It forces your body to fake diabetes by messing with your insulin levels. A sugary food will make your body produce insulin, then the sugar fizzles out and you're left with insulin, which makes you crave sugar... so you eat sugar, and the craving goes, but more insulin is made... See where this is going? Once you are totally in control of your sugar craving (it only took me about a month), you can have a small chocolate without feeling the need to devour the whole pack.
7: Cut out alcohol. Yes, I'm evil. No chocolate, and now no booze.
8: Don't buy anything which will not help you lose weight. Allow no crisps or chocolate in your kitchen. Buy fruit and vegetables to ensure something good is on hand if you get the urge.
Lots of people (particularly people who want to sell you diet food) will tell you that you don't have to "give up the foods you love". Sure. You don't
have to. But the foods you love got you where you are, just as all that pizza, chocolate, KFC and lasagne got me where I am. And believe me, the more you keep having something "naughty" now and then, the more you crave it. "Saving up your allowance" also kids you into maintaining your taste for unhealthy food.
The first couple of weeks will be hard. Your stomach is large, and it wants to be filled before it'll tell you it's not hungry any more. But you're in control, and you need to shrink it. Eat fruit. Eat vegetables. Go hungry if you reach your daily calorie limit. Your stomach'll get the idea very quickly and start shrinking back down to where it needs to be. A month in and you'll find that far smaller meals are actually very satisfying, and that even if you do get cravings, you can distract yourself from them.
You can do this. Do not doubt that
