How can I treat this? Low concentration?

From childhood I have had low concentration. Whenever I sit down to do something I often times get distracted by other thtoughts and become obsessed with them to the point that I need to find out more about it for eg while I'm working I'll think about cat herpes and then I'd have to research about it. Also sitting to study theory by myself is a task that I have difficulty doing bcs of how distracted and zoned out I sometimes get. I often times (it has reduced since coming to college) zone off lectures and start thinking about other things unconsciously and then catch myself zoned off. I also have to read a particular content more than once to register it. Till about 2 years back, I would often start enacting scenarios in my head but then find myself saying it out loud as if talking to myself. Why do I get so distracted and have such low concentration?