I think getting things out in the open on the forum is a good start. You obviously realize how screwed up u acted. Now that u accepted that you did wrong, you can start fresh. Start learning PU basics. Act like u never had a girl before. Do not take all the baggage of that last screwup. Accept your FAILURE and proceed to become a real man.
Also, you may definitely want to seek professional help. This girl is less an EX than a fixation or addiction.