Is there an addon I can use that will do the following:
1) Show me the spell's icon when I cast an instant spell.
2) Show me the spell's icon when I successfully complete casting a spell.
3) Show me something like 'the spell's icon with a red X over it' when my spell is interrupted (including by movement or /stopcasting)?
Ideally this'd be a horizontal stream of icons flowing as I do things, but even just seeing the last one or two would be helpful.
I'm pretty sure that most floating combat text addons can be configured in a somewhat resembling way.
You can try this one or that one.
Otherwise, WeakAuras should do the trick with some custom triggers.