Drop it

'Drop it' is something you say to end the topic of a conversation. Said as a direct command, it's most likely to be said by a parent to a child. Where the power relationship is closer, it is often asked, 'Can't we please just drop it?'

Examples:
-Mom, please can we stay up until 10? It's Saturday...!
-I've already said no, so drop it.

-Your aunt drives me crazy!
-I know. I'm sorry.
-I mean, she's always...
-Can't we please just drop it? We're almost to her house and I'm sure she's made a nice dinner for us!

It is what it is.

'It is what it is' is an expression someone will say when someone else has complained about a situation, and the person realizes that there's nothing they can do to change it. By saying this, they're trying to get the person who complained to realize it.

Example:
-I wish they weren't such micro-managers. It drives me crazy.
-It is what it is. I doubt they're going to change at this point.

Suck it up

'Suck it up' is used when you want to be supportive or tell someone to get over it when they're in an unfair or frustrating situation that they can do nothing about.

Example:
-it's not fair!
-I hear you. But I think you're just going to have to suck it up. Sorry.