Get over it
Don't concern yourself with something that's already in the past; accept it and move on to more productive pursuits.
We have been told to 'get over' our problems for centuries; for example, in Thirty-six Years of Seafaring Life, 1839, we have:
"Such was his state, that no one supposed he ever could get over it." [an amputation]
Something changed in the USA in early 1990s though and 'Get over it.' began to be used as a single sentence.
A pre-cursor to 'get over it' as a standalone phrase were a spate of articles relating to the USA's ongoing pre-occupation with the Vietnam War. In an article in the Kingston newspaper The Gleaner, October 1990 there was a report of a meeting between US Secretary of State James Baker and Vietnam's Foreign Minister Nguyen Co Thach. Following the meeting a diplomat said:
"The Americans have a serious emotional problem when it comes to Vietnam and the war. They have to get over it."
The single-sentence form came soon after. The earliest citation I can find is from a syndicated article by Anna Quindlen titled 'Give Hilary a role that suits her ability', which appeared in various US newspapers in November 1992 (co-incidentally, also involving James Baker):
"Breaking new ground is never easy, and Hilary Clintom surely knows about the people who said they wanted to 'get the pants off Eleanor and onto Franklin.' (Gee how times have not changed.) There will be people who complain that they didn't elect her. Get over it. You didn't elect James Baker either."
The phrase was unofficially adopted by the gay community as a part of the slogan "We're here and we're queer - get over it".
See also, 'get used to it'.