Most Helpful Girl
If you had reason to accuse her, then you aren't wrong.
If she showed no indication of cheating, and you accused her due to your own insecurities then yes you owe her an apology.
"How do I get her back?"
Start by admitting that you were wrong and then take it from there.
If she is still angry and wants some time alone... give her that space.
Just continue to let her know that she didn't deserve that and you will not do that again.