Just keep practicing it outside of match play. Have someone feed you higher than average balls to your BH and keep at it. Or, practice with a ball machine. or, practice against a wall. You'll figure it out, you just need a lot of time doing it.
How do you think guys like Federer are so good at so many awesome shots (aside from just a ground stroke)? They practice and isolate those shots for hours on end.
In a given match, how many of those slice BH's are you going to encounter? How many of them are above your shoulder? Not a whole lot, i don't think. Until you've done it 500/1000/10,000 times, then both your mind and body aren't used to it, aren't used to anticipating that shot and then going for it.