Rule #1: Slow work is SLOW
A note for those of us playing with very experienced fencers. Many of us have keyed up our reflexes to respond to threatening actions; a quick thrust is such an action. Sudden bursts of speed against an experienced fencer increases the likelihood of you being harmed.
Rule #2: Slow work is for learning
You're operating in an environment where your various senses can take in exactly what's happening.
So turn your senses up to 11 and try things and allow yourself to make mistakes.
Rule #3: Slow work is not about winning
This is not a competition. If you want to know how to defeat someone moving at 1/4 speed, simply play at full speed. Remember Rule #1 and Rule #2 (and Rule #1!!).
Rule #4: Slow work doesn't end until someone wins
If you make a strike (slowly!) and it doesn't fail to connect that doesn't mean the pass has ended, far from it. You should be immediately be continuing to keep yourself covered and continue the (slow!) "fight".
Rule #5: Know when you've been struck
This applies particularly when you're working without masks. If my partner stops their blow 2 feet from my face, they missed. If they stop it 6 inches from my face then their strike was in good measure; they pulled it short to avoid actually hitting me in the face. Recognize that, at speed, the strike would've hit you in the face and continue working.
Rule #6: Know when you've struck
The other side of Rule #5's coin is realizing that stopping your blow before striking your partner's face was actually a successful blow. Congratulate yourself on your excellent control and continue working.
N.B. This doesn't apply to strikes other than to the face/throat; you should be making contact.
Rule #7: Work on no more than 2 things
It's simple to want to work on all the challenges we have to overcome when fencing. It's also nearly impossible. Rather than trying to work on everything, pick one of two things and really focus on them. Make them clear goals too; "I want to improve my true fight" actually encompasses a huge amount of material. "I want to stop leaning to the side in seconda", on the other hand, is much more reasonable.
There's also nothing saying that you can't manufacture situations. If you want to practice staying upright in seconda then work with your partner to create situations that put you there.