Hi Sally,
My only thought towards this is that what you are feeling is completely natural, What has happened its done now I believe you shouldnt be crying over spilt milk, To err is human !!! Im definately not supportng what you did whether knowingly or unknowingly but dont get consumed by your past. If you are really sure that you love him try talking to him once about it and how you currently feel provided you are sure that he loved you while you were in the relationship. If he really did, forgetting the love of your life isnt that easy, he would try and understand and oblige with your apology and maybe he thinks he has 100 more reasons to give you one more chance against the one reason that you cheated on him.
If he doesnt then he’s way past it and you should move on too with a lesson and try and fall in love for real this time and focus on the one you love. Such is life and you will get over it soon.