Those who seek Self realization must pursue surrender or self-inquiry at every waking moment. In this effort, many seekers complain that they become forgetful and that their attention wanders. They want to know how to increase their concentration. It's not a matter of increasing concentration.
Forgetfulness happens because of an attachment -- there is an outcome the seeker values or fears, and feels they must focus on to get or avoid. That is what distracts them from surrender or self-inquiry, and it can manifest as forgetfulness. The attention wanders because it *wants* to wander, because there is something that it feels that surrender or self-inquiry will distract it from... and this is what is going on. There is an internal resistance. Just watching and recognizing these resistances helps loosen them.