honestly, there are more people who never make it all the way to their goal than there are those who lose too much, but that said, you can always increase your calories to either stop your weight loss or gain a few pounds if you think you're getting too thin. So I wouldn't worry about that at all.
I lost 100% of my excess weight and yes, for a time I was "too thin", but I was also aware that a majority of us have a 10-20 lb rebound weight gain during year 3. And I did. In addition, even before the rebound gain, things (fat, I guess?) tends to re-distribute itself after a big loss, so even though I weighed the same for awhile, I started looking a lot better. My face filled out, etc - even before that gain.
as far as it being hard - the first year, it wasn't too bad - esp before my hunger and appetite came back at month 5. For those first few months, for the first time in my life, I wasn't thinking about food at all the time. In fact, I didn't think about it much at all - and at times didn't even want any. Even though it was weird not to ever be hungry and to not give a flip about food, I found it very liberating! But things changed at month 5 when suddenly my hunger came back (and it does for most of us sometime during the first year). Then you're dealing with the hunger beast again, although it's not as strong as it was before surgery. But I wanted more than anything to lose this weight once and for all, so I really stuck to my program and lost it all (weight loss didn't stop until I was almost two years out, though - I had A LOT to lose!)
it does get harder after the first year because diet fatigue sets in, as well as some old habits. You really, really have to watch yourself. And eight years out, I have to continue to watch myself. It's so easy to slip into old habits. But I have a "drop dead weight" limit - I weigh myself once a week now, and if I hit that limit - or come close to it - I cut back until my weight is safely within the limit again. But this really isn't any different from most of my never-been-obese friends. So many of us have to monitor our weight and to cut back when it starts getting too high.
anyway, yes, it can be hard - esp when your hunger comes back and your motivation starts to wane, but unlike before, your effort actually pays off. You put in the work, and the weight comes off (or stays off). Before, the most I could ever lose is about 50 lbs (and even that was rare - most of my dieting ended up with a 10 or 20 lb loss). And inevitably, it will all come back. Happened every time. BUt this time, it worked!