Just because she's s intelligent and well educated, it doesn't mean she won't t do stupid things. If she's s sick again, that might affect her ability to think and act rationally too. It's s got nothing to do with academic intelligence or education, IMO. There's plenty of intelligent people do daft things, especially under pressure. Belle has always been highly strung and given to doing wayward things and the incident with Gemma, causing her to feel immense guilt and fall ill seems to have exacerbated that. I think it was easy to cast her in the mould of sensible, smart, precocious kid when she was younger but she's old enough to strike out on her own and become independent now, and she's often panicking about things, losing control and making mistakes. After all, her ex GP boyfriend is presumably level - headed and intelligent, and he's s given up everything to shack up with a troubled, mentally ill girl twenty years younger than him!