I don't know how serious your husband's heart condition is, but you shouldn't need to walk on eggshells. Perhaps your gp (or his, if they are different) could help with some advice? Perhaps it will be easier to cope as you both become more accustomed to the situation?
Serious anti-depressants are invaluable, but only for clinical depression (ie being depressed without adequate cause, which by no means describes your situation) but you might find some herbal calming products helpful - a Boots pharmacist could advise you, or your gp if she is sympathetic.
If you force yourself to be calm and cheerful when all around you are people being grumpy and demanding you will end up having a heart attach yourself, and then what will become of them?
Good luck.