I have been married for 26 years, have never cheated on my wife, never hit her, but have made some huge mistakes in our relationship. There have been a few times that I have been sure that I had wrecked our relationship beyond repair.

Just talkign to her, telling her that you are sorry, that you want to make things better, and that you need her help to do it. Those few words, if you really mean them, won't make some instant change to make things better, but they are a step in the right direction, and with work, hope and love, a relationship that might seem beyond repair, can be patched up. There is no way to completly repair serious dammage to a relationship but it is possible to fix it up enough, and given time, and with mutual trust, the remnants of the dammage done will still be there, but you can do other things to make your relationship stronger than it was, stronger than it has ever been.