Are you using bumper cam ? Imo a mirror is crucial, with it you can actually see them coming most of the time. You dont have to actively look in the mirror just scan for movement in your peripheral vision.
About dealing with them, i go super defensive on them. Defend the inside as soon as they get in divebomb range, never let them get on your inside, if they do just try to cut back, but whatever you do dont position yourself on the outside of them in a corner, its to much of a risk. This is gonna cost you some time but the strategy is to keep them behind long enough (hopefully just a few corners) untill they mess up or you get just out of striking range, then you go full timetrial mode to drive away from them if you can, if you cant and they catch up you repeat or decide maybe its better to let them go, depending on the situation of the race.
Here's a nice example of a driver i raced yesterday, wont mention his name but this clip from his view shows how i try to defend from him.
It starts when he punts me on corner entry coming from way back. This immediately made me lose all trust in him, notice how i immediately go onto a defensive line in the hairpin, i wouldn't do that with such a gap with a driver i trust. That incident on s/f showed i was right not to trust him as he almost kills us both in a desperate attempt to get on my inside. I continue to defend the inside with succes untill the last lap. Notice how i go a little wide on entry and this leaves just a little space on the inside ? He immediately drives into that space and now i i am vulnerable on the outside and lucky to not crash into the wall. He won that race, and though you could argue that that sucks cause he was not the cleanest driver i should have never left him that space.
