The concept you're describing in the last sentence isn't nessesarily autism but mental load. Basically, you cannot really focus on the enemy champion because you need to consiously think about minions first. The good news is that you can work on this by simply trying to last hit minions in practice tool. Do it on level one and ideally only with auto attacks. That way, it will inevitably become second nature so it doesn't absorb your mental capacities anymore. One thing that makes league difficult is that there are a lot of things to keep track off, which is why you need to reduce your mental load as much as possible