You haven’t mentioned the mistake but if you are sure you love her you should give her time because it takes time for one to analyse and make decisions that are right. It will hurt but know that pain demands to be felt and love is sacrifice. You mention not being able to concentratebut let me suggest that you write a diary or something maybe dedicated  to her and openly say everything in it.it will help you calm down and take your mind off it.

I read somewhere that love is when you come to realize that the time you spent with her seems like a blessing. So be thankful that you both were lucky enough to share this love for 5 years and bad times are followd by happy ones so be patient. Hope you feel better soon.