I'm the same as you, I've got ADHD and I'm a caster main as well, viper is honestly pretty easy to learn, theres only 2 positionals and they're at the end of the combo and you just go back and forth, just remember red is rear, green is side, I also recommend setting up your hotbar to make positionals make sense. Aside from that just practice, make viper your new main and it'll become second nature over time. That's honestly the best advice I can give as someone in your situation that picked up viper.