obsessive

An obsessive person is someone who can't stop thinking about or doing something, even if it's causing problems for themselves or others.

People often use this word to describe behaviors like checking their phone repeatedly, constantly worrying about a particular issue, or being fixated on a certain hobby or activity. You might hear someone say "I'm starting to get worried that my friend is becoming obsessive with their new project – they're spending all their time working on it and ignoring everything else." In this sense, the word isn't necessarily bad – it just means that someone has become very focused on something, maybe a bit too much. Obsessive can also be used more negatively to describe a pattern of behavior or thought that's becoming unhealthy or intrusive, like constantly thinking about an ex-partner long after they've broken up with them.