Huggy I actually only realised how heavy I'd once been because I found an old weighing-in slip from one of those BMI machines at the gym, I was pretty shocked, I thought my heaviest had been half a stone or so lighter than it was
It's funny, I remember when I did that weigh-in, I knew I'd gained weight and wasn't happy about it (hence being at the gym) but I didn't have any real notion of what weight I was aiming for - I only established my goal weight later, by going to the doctor's surgery and asking them how much I'd weighed when I signed up. I also had hilariously unrealistic expectations of how quick and easy it would be to lose if I just put my mind to it...!
In terms of staying motivated, we used to have a refrain on this thread, "WHY IS IT SO HARD AND BORING?" - keeping motivated is half the battle. For me, the answer has been to design a totally rigid way of eating (no sugar / carbs / booze Mon-Thurs, whatever I like Fri-Sun), so that the mental challenge is just gone. I pretty much just know what I'm going to eat every day when I wake up, so it isn't a constant exhausting negotiation with myself. There are still challenges, obviously, but after all this time, there's really 0 chance of me randomly having pizza, wine and ice cream on a random Wednesday, unless I have some specific reason to have "scheduled" it in (like MrQuim's birthday last week).
So that's really a way of saying, I can't do it if I rely on motivation! The big thing for me is having a "release valve" built in - I don't obsess over things I can't have on the days I can't have them because I know when I can have them. I don't know how people can deny themselves indefinitely, whenever I tried it it drove me barmy, but I know that lots of Slenderistas find that if they let their hair down on the weekend they can't find the wagon again on Monday (not to mix metaphors or anything...!) so it's completely about your individual psychology.
I think that ^ is why people find the 5:2 so useful: when you know it's a fast day there's just no point thinking about it, and the stakes aren't all that high, as if you're ferociously craving something, you know you can have it tomorrow. It's a slow and steady plan (for most people), but it's easy enough to stick to, and it gets easier and easier in time, so if you just trudge along with it, you'll generally find yourself several kilos lighter within a few months without it having felt like a constant grind.