Easy
save, rescue, protect
tough
This is a tough one to answer
Tender meat
The word that means almost the opposite of "in haste" is "deliberately." While "in haste" implies acting quickly and without careful thought, "deliberately" suggests a thoughtful, intentional approach, taking time to consider actions before proceeding.
I am guessing that you mean tough. Antonym for that depending on your meaning would include: weak, malleable, compliant.
The simple ones
Cluster means a close grouping of things, so yes it would be considered an antonym. Words it is opposite of would be: one, singular, etc.
The opposite meaning of sissy would be tough, strong, or masculine.
A easy question
Retain
To be mentally tough means to be tough to the mind.