Is it a fallacy to say, "I can't be happy under those circumstances"?

This is one that highly depends on the surrounding circumstances. For example, it needs a "because..." to evaluate the fallaciousness. Without getting too psychological, there are situations where people have very little control over their happiness—especially people who have a difficult time internalizing situations and finding the "silver lining." So if someone is aware of their tendencies and how they felt under similar circumstances, it would not be unreasonable for them to assume they cannot be happy those under circumstances, a bit pessimistic perhaps, but not unreasonable. Now, if a reason was given such as "because person X said he can't be happy under those circumstances and person X is a master astronomer," that's a different story. :)