Is there any way to diminish this? Another player told me it was luck based but my luck must be very high because the gods are fond of me, yet the longer I play the more frequently I lose focus upon casting spells, which leads me to suspect that there must be some skill/attribute controlling this that I am ignoring. Any ideas?

Bad luck(not the stat) and rng. As long as ur luck isn't negative I'm pretty sure it shouldn't make you lose focus.

I have always found balance effects losing focus. If your spells are too far above your BWIAS or visa versa. I have also felt balance between the 3 main stats for each spell can effect focus. I may be way off base as I have never read the code, but I have spent some time running Haras.

I can not see the code from here but from recollection it is purely random for Bless, Protect, Enhance, MS and GC.
However, I have noticed that random reflects life and spells fail when you need them most.

My suggestion is the carry 2-4 potions and lots of purple and a few red flowers.
At low ranks, potion bottles do not disappear and you can refill them with the flowers.

Some spells modify the chance of losing focus based on the difference between the casters skill and the target's resistance or the power of the spell cast on you already (the target may also be the caster in some cases)
- Curse
- Corrode
- Identify
- Stun
- Dispel
- Compulse

All the others have a random chance of failing.
Curiously GC has a higher chance to fail.
The chance to fail for Assassinate was removed.

Reviving this as it became a bit absurd while in Abandoned Town Generals.
It takes me an average of 5 attempts to curse these skeletons.
Sometimes I lose my entire mana bar due to focus loss.
I'm a Baron with maxed out spells and stats.

As a warrior I'm not sure the expectation should be held that your spells will be very good. I don't see them as viable spell casters, unless you have a SS set or three.

Yes as a non rb warrior it's really hard to land curse. Maybe if you can find a shield with curse that would help. Also equipping rops en prison gear will boost it up just a little more. Just focus on a curse spellset so you can switch back to fightset after it has landed. That's how I did it.

Greetings

*edit* a Dpot would also help