If you get a chance to apologise you have to stick with honesty now. You've said it and it's done. Focus on the positives. There is a reason/s you are/was with her. Focus on that.

At the very least maybe the break up will be less of a sting. At best maybe she sees that there is still something that can be worked on.

Ultimately though, you fucked up. If you get another chance take it but show her some respect on what she decides to do from here.