When you decide you want to get back with ex, you must be ready to face not only what has gone wrong with the relationship, but also your part in the break down of the relationship. If you made mistakes then don't beat yourself up for that. We all make them, it's what you do after the mistake that's important.
So sit down on your own and take some time to go through what happened and what were the issues that led up to what happened. Don't concern yourself too much with what you see as your ex's mistakes, leave that for the future. Only concern yourself right now with what you can influence.
Whatever mistakes you find in your past behavior, can they be forgiven? Don't be too eager to get down on yourself and believe that your mistakes are basically unforgivable. You would be surprised at the situations that couples find themselves in and that they later recover from. So don't think that your mistakes are beyond forgiveness. There is always hope!
Try and pin point what was happening in your life at the time that led you to make the mistakes, especially if it was completely out of character for you. Remember what I said about facing hard truths, if you want to get back with ex, then you need to do this.
You might discover that you were under certain stresses and strains. Perhaps you were under threat at your job? Or you weren't physically feeling yourself? Or perhaps there were other family issues. Whatever you find when you look back at what happened, you will eventually need to sit down with your ex and explain in detail just what you have found.
If your mistakes were the result of you just being thoughtless or selfish, then this is something about you and your character that can quite easily be fixed. It will take ongoing monitoring but if you are sincerely committed to get back with ex, then it is going to be worthwhile.
When you have come to the point where you have begun to gain perspective about your mistakes, then call your ex and arrange to meet. It is usually better to arrange a face to face meeting somewhere public and neutral because then there is less chance of either of you becoming emotional.
So, make sure that you stay calm and focused on what you want to say. Apologize and ask for forgiveness and then listen carefully to what your ex has to say.