You're in luck because all you have to do is keep lapping and practising.
Consistency is the key. And it can only come from learning all brake points and doing lap after lap after lap.
When I started playing ACC I was probably at least 2 seconds slower than I am now. It's mostly thanks to following video advice from ACC forum members and devs and the ratings in ACC itself (my load cell pedals probably helped a little too). I seem to be more or less on par with SlowGrayAudi now (1:31:5 on Zolder practise session in Fast conditions) and 3-4 second behind aliens on many tracks. More on some tracks.
But I wouldn't worry about aliens because as you said, there's always somebody faster.
Worry about being consistent and not overdriving the car.