I have habits that I don't have to think about and then one day they're just gone and I have to start over. Usually it happens by having a sudden realization that the thing I did for months without thinking about it has not been done for several weeks. Then to rebuild that habit I have to start over again.
Makes sense. I never had any habits besides getting various forms of entertainment on the internet so I can't relate. I always have to make an active effort to do something and then at some point (usually days, a few weeks is my absolute maximum) my brain refuses to even do the bare minimum to make me do stuff.