"Tonight was my girlfriend and I's final mixed match"?? If you're going to violate rules and jump to the other side of the court, let's at least be literate. When in doubt about multiple subjects, eliminate one >> "Tonight was I's final mixed match." Sound good? Didn't think so >> "tonight was my girlfriend and my final mixed match." Correct, but akwardness is best handled by rewriting >> "Tonight was the final mixed match for my girlfriend and me." That's better!!
"Where's the REST of me?"
Ronald Reagan, in "Kings Row"